1、第三讲理清文架构核心抓段落构成知主题句一、段落的构成英语文章段落结构一般由三部分构成：主题句(topic sentence)，扩展句(development sentence)和结论句(closing sentence/concluding sentence)。主题句点出段落的主题，扩展句解释说明或论证主题，结论句总结、归纳或概括主题。1主题句主题句，就是段落的核心句、纲领句、主旨句、中心思想句，言简意赅，主旨鲜明。它是段落的灵魂，表达段落的主题，在英语文章段落中居于主导地位，其他句子都要围绕它而展开。英语文章一般喜欢开宗明义，所以主题句一般位于文章段首，但有时也在段中或段尾。2扩展句扩展句就
2、是为主题句起解释说明或论证作用的句子。扩展句一般长度适中，有一定的条理和逻辑。扩展句的展开方法多种多样，通常采用的方法有举例说明、描述、定义、比较对照、因果分析、分类等。3结论句结论句就是总结句，会重述或总结段落的中心论点，与主题句相呼应。需要指出的是，英文段落中结论句并不多见，我们通常看到英语的文章段落结构60%一70%都是由“主题句扩展句”构成。典例Why fiction is superior to textbooks lies in their imagination(主题句：以自问自答的方式提出段落主题)Authors of the fictions always create at
3、tractive characters so as to give his opinion about nature and the society as well as life(扩展句1)It not only provides knowledge，but also teaches us what is the right attitude toward life (扩展句2)For instance，we can learn from Gone with the Wind how to be a strongwilled person；from Jack London，how to lo
4、ve life(扩展句3：基于上述观点，进一步举例说明)We can be stimulated to overcome all obstacles to realize our ambitions(扩展句4)So，fictions enable people to form their positive view of life，the spirit of optimism and invincible courage，and provide us with an opportunity to learn and to practice，while textbooks barely can
5、(结论句：归纳总结段落主题)二、段落主题句的位置 在进行阅读理解时，我们要善于找出段落的主题句，这样才能抓住文章段落的中心思想(controlling idea)。主题句在段落中的位置并不是一成不变的，而是根据作者组织段落的方法或强调的重点而定。大多数情况下，主题句在段首，有时会在段中或段尾。无论主题句的位置在哪儿，主题句都包含一个逻辑性强且层次分明的中心思想。1主题句在段首这是主题句在段落结构中最常见的位置，这种安排开门见山，点出主题，读者可以迅速了解文章主旨。典例Although smoking has been a popular habit for centuries，it has a
6、lways had its enemies(主题句)Some American Indians advised young braves not to smoke too much because tobacco would cut their wind(削弱力量)in a hunt or a battle.King James of England called smoking “a branch of the sin of drunkenness，which is the root of all sins”Ben Johnson，the playwright，said smoking wa
7、s“good for nothing but to choke a man and fill him full of smoke and embers”Popular slang called cigarettes“coffin nails”long before the medical evidence on the hard from smoking was complete.Now every package of cigarettes and every advertisement for cigarettes must carry a warning from the Surgeon
8、 General(卫生局局长)that smoking is hazardous to health.2主题句在段中 主题句出现在段中时，位于主题句之前的部分通常起到承上启下的作用，而主题句后面的部分则围绕主题句展开讨论或阐述。此类主题句并不一定在段落正中位置，可能靠近段首，也可能与段中的结论句相邻。典例My parents have gone out for the evening.Just as I settle down to read or watch TV，my little brother demands that I play with him.If I get a teleph
9、one call，he screams in the background or knocks something over.I always have to hang up to find out whats wrong with him.Babysitting my brother is no fun(主题句)He refuses to let me eat a snack in peace.Usually he wants half of whatever I have to eat.Then when he finally grows tired，it takes about an h
10、our for him to fall asleep.3主题句在段尾主题句位于段落的结尾，是对段落主旨进行概括和总结。主题句位于段尾的好处是会给读者留下深刻印象。典例Some people dream of starring roles，their names in lights，and their pictures on the cover of magazines.Some would even go out of their way to get their name.However，I am not one of these people.A famous person gives u
11、p private life，feels pressured all the time，and is never completely safe.So，let someone else have that cover story，take the starring roles and have a big name.Id rather lead an ordinary，but calm life rather than a stressfilled public one(主题句)理段落展开析句间关系在段落中，般会有两个甚至多个扩展句对中心思想进行论证、解释，这也就是段落的展开。段落的展开模式多
12、种多样。这里我们着重介绍9种段落展开模式。列举型列举法是比较常见的段落展开法。按此类方法展开的段落中，作者会先点明主题，提出论点，然后列举一系列的论据进行陈述或解释。列举的内容可以是一系列的事物、事件、理由或者一个问题的不同方面。列举的顺序可以是时间的先后、地理位置的远近或者内容的相对重要性等。典例The sentences in most well written paragraphs may be analyzed into three general functions.First，there are paragraph introducers，which are sentences t
13、hat establish the topic focus of the paragraph as a whole(列举1)Second，there are paragraph developers，which present examples or details of various kinds that support the ideas set forth by the paragraph introducers(列举2)Third，there are paragraph terminators，which logically conclude the ideas discussed
14、in the paragraph in a psychologically satisfying manner(列举3)Not all pieces of writing will conform to this analysis；however，most successful paragraphs usually contain the same combination of these three sentence types.举例型举例法是用具体的事例来说明段落主题，也是一种常见的段落展开方法。此类段落中，作者往往开门见山，点出主题，然后列举具体事例(一个主要例子或多个相关例子)进行说明
15、。所举事例一般较具体，可以是日常事件、现象、个人经历、名人轶事等等。典例The ability to write wellorganized，concise(精确的)paragraphs is essential(必要的)to a students success in almost all university courses.In preparing scientific reports of laboratory experiments，a student must present his or her findings in logical order and clear langua
16、ge in order to receive a favorable evaluation(评价)of his or her work(举例1)In writing successful answers to essay questions on history or anthropology(人类学)examinations，a student must arrange the relevant facts and opinions according to some accepted pattern of paragraph structure(举例2)And certainly in w
17、riting a book report for English，or a critique(评论)for political studies，or a term paper for sociology，a student must pay attention to the style and organization as well as the content(举例3)Clearly，skill in expository writing(说明文)is crucial to successful achievements in most university subjects.描述型段落也
18、可以用描述的方法来展开。常见的描述法有三种：时间顺序、空间顺序和过程顺序。这三种模式使段落的层次体现了一定的次序关系。用这种模式展开段落，作者能够清楚连贯地交代事物的本末，读者可以容易地理解段落的主旨。1时间顺序这种方法主要是以事情发生的时间顺序或先后次序来展开段落。常出现在描述历史、时事、人物或事件的发展过程的文章中。描述顺序有“顺序”、“倒序”和“插序”。典例Bill Gates stepped down as chief executive officer of Microsoft in January,2000.He remained as chairman and created t
19、he position of chief software architect.In June,2006，Gates announced that he would be transitioning from fulltime work at Microsoft to parttime work，and fulltime work at the Bill & Melinda Gates Foundation.He gradually transferred his duties to Ray Ozzie，chief software architect，and Craig Mindie，chi
20、ef research and strategy officer.Gates last fulltime day at Microsoft was June 27,2008.He remains at Microsoft as nonexecutive chairman.(顺序)2空间顺序按照空间顺序展开的段落主要见于描述场所或空间、人或物的空间位置及其相互关系的文章中。典例It is a beautiful and quiet night.The moon is like a disc hanging in the dark sky，which casts its light on ever
21、ything.Around the lake are some trees，their slender leaves blowing in the gentle wind.There must be some frogs in the middle of the lake，whose endless singing can be heard clearly.What an attractive night！(由上及下，从远到近)3过程顺序过程顺序法主要是描述做一件事情应该遵循的程序或步骤。这种段落展开的方法多见于培训手册、科技文章和操作说明书中。典例The ancient pyramids o
22、f Egypt continue to fascinate and amaze people nearly five thousand years after the Egyptians built them.Constructing each massive(巨大的)pyramid was difficult and dangerous labor，requiring thousands of workers and years of toil.First，heavy limestone (石灰岩)blocks were taken from quarries(采石场)located nea
23、r the Nile River(步骤1)Then the blocks were transported by wooden boats to a point as close as possible to the intended building site of the pyramid(步骤2)Next，the blocks were unloaded and slowly hauled to the actual site by means of sledges(重型运输雪橇)positioned atop wooded rollers(滚筒)(步骤3)As construction
24、proceeded，the Egyptians moved each successive(依次的)block into position using ramps(滑轨，斜坡)made of bricks of dried mud(步骤4)Incredibly，the huge pyramids were completed without the aid of bulldozers(推土机)，cranes(起重机)trucks，or other modernday construction equipment.Even so，the tallest pyramid rose higher t
25、han a fortystorey building！Equally remarkable，numerous pyramids still stand today，almost fifty centuries since their construction.比较对比型比较对比法是将两个可比的项目进行比较以找出其相同点或进行对比以找出其不同点。比较或对比的方法有两种；一种方法叫整体法，即先讨论A的特点，再讨论B的特点，即A1，A2，A3；B1，B2，B3。另一种方法叫逐项法，即将A的特征与B的特征交替比较或对比，即A1，B1；A2，B2；A3，B3。1整体法典例Laws and ethics(
26、伦理)are not quite the same.In general，laws are societys attempt to formalizereduce to written rulesthe general publics ideas about what is considered right and wrong conduct in various spheres of life.However，it is rarely possible for written laws to capture all of the subtle shadings that people inc
27、lude in the codes of ethics they use to govern their lives.Ethical concepts(伦理观)，or moral principles(道德准则)like the people who believe in themare more complex than written rules of law.Ethical concepts are ideas about right or moral conduct，and they cannot always be expressed in the formal language o
28、f law or in rules.(A1，A2；B1，B2)2逐项法典例In college and university courses，the objective test and the essay examinations are two contrasting methods of evaluation commonly used to measure a students grasp of subject matter.The objective test usually consists of a large number of unrelated questions that
29、 require the student to demonstrate mastery of details.It often leads to rote memorization(死记硬背)of isolated facts during the pretest study.Since the questions on the objective test are presented in truefalse or multiple test form，the student may be encouraged to guess answers for which he has no acc
30、urate(准确的)knowledge.The essay examination，on the other hand，usually consists of a few broadly stated questions that require the student to give proof of his ability to handle general concepts.This type of examination also relies on factual information，but there is far greater necessity for the stude
31、nt to demonstrate analytical and compositional skills.Mere guessing at answers is reduced to a minimum.Although the objective test and the essay examination have similar goalsthe assessment of a students academic achievementthe techniques (and very often the results) of the two types of examination
32、differ significantly.(A1，B1；A2，B2；A3，B3)因果分析型因果分析法是通过分析事物发展的原因及其结果来说明段落的主题。无论是说明原因还是阐释结果，都有行文逻辑可寻。此种方法通常有两种模式：原因在前，结果在后；结果在前，原因在后。1原因在前，结果在后典例According to statistics，China has one of the highest traffic fatalities in the world，and alcoholrelated traffic fatalities had been increasing and had very ba
33、d influence on the society(原因)Because of this，stricter punishment had been imposed on drunk drivers，and stricter laws had been passed on drunk driving(结果1)As a result of this，drunk driving and alcoholrelated traffic fatalities have been on the decline(结果2)2结果在前，原因在后典例During the past five years，the n
34、umber of Americans killed annually in car accidents has climbed to more than 55,000(结果)This needless slaughter(屠杀)on streets and highways can be attributed to(归因于)three general causes.Mechanical failures，especially those related to faulty brakes and bald tires，account for a significant number of fat
35、al(致命的)accidents(原因1)Environmental conditions such as blind corners，narrow streets，heavy fog，intermittent(间歇的)rain or snow resulting in slippery roads also contribute to the accident statistics(原因2)But without doubt the most frequently reported factors in car accidents are errors of human judgmentsa
36、ll the way from such follies as excessive speed and drunk driving to such momentary lapses(失误)as failure to signal a turn or a change from one lane to another(原因3)The man behind the wheel is often his own worst enemy.分门别类型分门别类，顾名思义，就是按照一定的标准，将某一复杂的事物进行分组归类。一种复杂的事物往往可以根据其性质、形态、成因、功能等属性分成若干类，然后逐一说明。这种
37、方法既可以展示事物的共性，又可以展示事物的特性，多见于科技类的英语阅读文章中。典例Individual paragraphsthe building blocks of essays，articles，chapters，and other longer papersmay be classified in a variety of ways.At the essay level，paragraphs may be sorted into functional groups such as introductory，developmental，transitional，concluding，an
38、d the like(分类1)Depending on the purpose or intent of the writer，particular paragraphs may be thought of as aiming to persuade，inform，argue，or excite(分类2)Paragraphs may also be classified according to such techniques of development as comparison，contrast，and definition(分类3)Another developmental devic
39、e might also be the classification paragraph，which organizes items or ideas to be discussed into relatively homogeneous(同种类的)groups(分类4)Such classifications make it possible to talk about a large number of paragraphs by grouping them into a small number of classes.定义型定义法是用简单而准确的语言描写某一事物的属性，或解释说明一个专业
40、术语或者某一概念，特别是抽象概念。这种方法常见于说明文、应用文或议论文中。一般来说，用定义法展开的段落包含两部分：首先是定义句，概述被定义物的性质；其次，展开叙述有关方面的内容，如来源、性质、结构、特征、原理、用途等。典例A rain forest，as the term suggests，is a kind of wooded area，subject to usually heavy and frequent rain (定义句)Found only in the tropical and subtropical(亚热带的)regions of the Caribbean，Brazil，A
41、frica，and Asia，rain forests contain a great variety of trees including bamboo，palm，cedar，ebony，calabash，and whitewood(来源)Many of these trees grow to a height of more than a hundred feet，with dense canopies(树冠)or crowns forming three storeys.Although ground plants are rare，climbers of all kinds aboun
42、d(充满)，and often exotic flowers appear in thick leaves.This luxuriant(茂盛的)growth results from an annual rainfall in excess of ninety inches，with no dry season(特征)概括型概括法与举例法类似，这两者都是用事例来说明段落主题。概括法强调结论是基于事例所说明的事实和观点而得出的。而且这一结论易于被读者接受。典例The surface of the earth is chiefly watersomething that we，as dwelle
43、rs(居住者) on the land，are apt to ignore or completely forget.As stated earlier，the Pacific Ocean alone covers nearly one third of the globe.The combined areas of all water bodies，including oceans，seas and lakes add up to nearly two and onehalf times that of all the land of the earth.In other words，abo
44、ut 71 percent of the earths surface is water.In addition to the large expanses(陆地、海洋等广阔区域)just mentioned，there are small ponds，waters that run as streams on top of the land，and other waters that lie or move within the upper portion(部分)of the earths crust(地壳)And there is water in vapor(水蒸气)and conden
45、sed forms in the atmosphere.Thus，water is an important and practically all pervasive(遍布的)element in mans habitat (概括句)综合型前面我们介绍了8种段落展开模式。然而在多数情况下，段落的展开方式多样而富于变化，常常是几种模式互相结合，互为补充，这种段落展开方式我们称之为“综合型”。典例1Increasing your vocabulary can help you in a number of ways(分门别类法)You discover that knowing synonyms
46、，for example，will decrease the amount of repetition in your compositions and make them more enjoyable to read(举例法)And if a teacher enjoys reading your paper，he may give you a better grade.Also，an increasing vocabulary will make your own reading more enjoyable.You will find it much easier to follow t
47、he ideas in your history text book and the newspaper when you do not have to continually run to the dictionary to look up an unknown word.You will become more eager to read and not as discouraged when you do come across an unknown word.As a result，you will read faster and more intelligently，become m
48、ore knowledgeable，and hence better informed about the world around you(因果分析法)典例2My album collection reached the 300 mark，so no matter what mood I am in，I have a variety of pieces to choose from.In recent years I have found myself buying more and more blues albums.I dont know why，but I have simply be
49、en enjoying them.I can listen to the blues in the morning when my head is still a little foggy，and the voices come through like lighthouses leading the way.Or I can listen to them after a long day and let that simple rhythm put me to sleep.When Im feeling bad，I put one of these albums on and sponge
50、up all the raw emotion expressed on it(分门别类法)From there I can progress in energy levels，from slow blues (Robert Johnson) to inspired blues (Ray Charles) to faster and upbeat(乐观的)blues(Taj Mahal) (举例法)In this way the blues actually inspires me；it is like a catharsis(情感净化)of emotion(因果分析法)析段落排列明主旨记叙文中
51、的“具体事例型段落”(抓住此事例围绕的观点)这种类型的段落整段都在讲具体事例。往往这个事例本身并不重要，而作者是为了证明自己的某个观点才讲了这样一个事例。因此，考生要站在宏观的角度把握这个事例证明了什么观点，或者说明了哪一方面。它的特点就是会有特别具体的描述词，比如某个人名。多次被提到，而且用到的动词也都是一些表示具体动作的动词，而不是观点动词。典例(2018全国卷阅读C节选)1While famous foreign architects are invited to lead the designs of landmark buildings in China such as the ne
52、w CCTV tower and the National Center for the Performing Arts，many excellent Chinese architects are making great efforts to take the center stage.2Their efforts have been proven fruitful.Wang Shu，a 49yearold Chinese architect，won the 2012 Pritzker Architecture Prizewhich is often referred to as the N
53、obel Prize in architectureon February 28.He is the first Chinese.citizen to win this award.分析 选文中第二段属于“具体事例型段落”。本段中使用了won，is referred to，is等一系列客观具体的动词和短语，以王澍为具体事例，介绍了王澍获得2012年普利兹克建筑奖，得到了国际上的认可。 目的是为了证明本段的主旨：Their efforts have been proven fruitful。说明文中的“对比型段落”(分清层次最重要)这种段落最常见的就是按时间分类说明或按某一特性分类说明。针对这种
54、段落，要特别留意段落中的过渡词，比如，However，but addition，finally等等。典例(2019全国卷阅读C节选)This development did not take place overnight. It had been possible(but not easy)to buy single copies of newspapers before 1830，but this usually meant the reader had to go down to the printers office to purchase a copy. Street sales w
55、ere almost unknown. However，within a few years, street sales of newspapers would be commonplace in eastern cities. At first the price of single copies was seldom a pennyusually two or three cents was chargedand some of the older wellknown papers charged five or six cents. But the phrase “penny paper
56、” caught the publics fancy, and soon there would be papers that did indeed sell for only a penny.分析从节选的这一段可以看出，这段说明按时间顺序描述了随着时间的推移，报纸是怎样走向街头，走向大众的，其中体现时间顺序的词语有before 1830；within a few years；soon等。议论文中的“总分型段落”或“分总型段落”(一定要找到并读懂主旨句)“总分型段落”排列在一开始就给出了作者的观点或者要说明的主要内容。后面的大部分都在论证这个观点或者进行细节上的说明。针对这种类型的语篇，考生应
57、该做到细读主旨段落，略读细节段落，以读懂主旨为要务。考生会发现文后的题目大都围绕主旨而命题，而偏离主旨的大都是干扰项，而不是正确答案。“分总型段落”排列在开始和中间都是一些细节型的内容，有时是平行的细节，有时是层层递进，但一直到语篇的最后一段才点出最重要的观点或结论。显然读这种语篇的落脚点应该在结尾处。跟总分型的语篇相比，分总型的语篇有时更耗时间去发现主旨，因此考生们平时做题的时候要多去体会，争取在最短的时间找到主旨。典例(2019北京卷阅读C)1The problem of robocalls has gotten so bad that many people now refuse to
58、pick up calls from numbers they dont know.By next year，half of the calls we receive will be scams(欺诈)We are finally waking up to the severity of the problem by supporting and developing a group of tools，apps and approaches intended to prevent scammers from getting through.Unfortunately，its too littl
59、e，too late.By the time these“solutions”(解决方案)become widely available，scammers will have moved onto cleverer means.In the near future，its not just going to be the number you see on your screen that will be in doubt.Soon you will also question whether the voice youre hearing is actually real.2Thats be
60、cause there are a number of powerful voice manipulation(处理)and automation technologies that are about to become widely available for anyone to use.At this years I/O Conference，a company showed a new voice technology able to produce such a convincing humansounding voice that it was able to speak to a
61、 receptionist and book a reservation without detection.3These developments are likely to make our current problems with robocalls much worse.The reason that robocalls are a headache has less to do with amount than precision.A decade of data breaches(数据侵入)of personal information has led to a situatio
62、n where scammers can easily learn your mothers name，and far more.Armed with this knowledge，theyre able to carry out individually targeted campaigns to cheat people.This means，for example，that a scammer could call you from what looks to be a familiar number and talk to you using a voice that sounds e
63、xactly like your bank tellers，tricking you into “confirming”your address，mothers name，and card number.Scammers follow money，so companies will be the worst hit.A lot of business is still done over the phone，and much of it is based on trust and existing relationships.Voice manipulation technologies ma
64、y weaken that gradually.4We need to deal with the insecure nature of our telecom networks.Phone carriers and consumers need to work together to find ways of determining and communicating what is real.That might mean either developing a uniform way to mark videos and images，showing when and who they
65、were made by，or abandoning phone calls altogether and moving towards databased communicationsusing apps like FaceTime or WhatsApp，which can be tied to your identity.5Credibility is hard to earn but easy to lose，and the problem is only going to get harder from here on out.行文脉络第一层(第1段)开篇由人们拒接陌生电话号码引导手
66、机欺诈现象。并指出“解决方案”被广泛使用时，不法分子就会转向更聪明手段。分第二层(第2段)新的声音处理和自动化技术可以给人们带来便利。第三层(第3段)这一技术让不法分子有了可乘之机。第四层(第45段)提出解决方案。总议论文中的“总分总型段落”(关键是弄清楚文章的论点及结论) 这种类型的段落排列是议论文常用的结构方式，“总分总”的结构是先抛出文章的中心思想，再分层叙述，逐层深入，最后对文章加以总结。即开头提出论点(开门见山)；中间若干分论点；结尾总括论点(或重申论点，或总结引申论点)，而几个分论点之间可以是并列关系、层递关系、对比关系等，但不能是包含关系或交叉关系。典例(2018全国卷阅读D)1
67、Adults understand what it feels like to be flooded with objects.Why do we often assume that more_is_more when it comes to kids and their belongings？The good news is that I can help my own kids learn earlier than I did how to live more with less. 2I found the preholidays a good time to encourage youn
68、g children to donate lessused things，and it worked.Because of our efforts，our daughter Georgia did decide to donate a large bag of toys to a little girl whose mother was unable to pay for her holiday due to illness.She chose to sell a few larger objects that were less often used when we promised to
69、put the money into her school fund(基金)(our kindergarten daughter is serious about becoming a doctor)3For weeks，Ive been thinking of bigger，deeper questions：How do we make it a habit for them？And how do we train ourselves to help them live with，need，and use less？Yesterday，I sat with my son，Shepherd，d
70、etermined to test my own theory on this.I decided to play with him with only one toy for as long as it would keep his interest.I expected that one toy would keep his attention for about five minutes，ten minutes，max.I chose a red rubber ballsimple，universally available.We passed it，he tried to put it
71、 in his mouth，he tried bouncing it，rolling it，sitting on it，throwing it.It was totally，completely enough for him.Before I knew it an hour had passed and it was time to move on to lunch.4We both became absorbed in the simplicity of playing together.He had my full attention and I had his.My little exp
72、eriment to find joy in a single object worked for both of us.行文脉络第1段在文章首段提出论点：how to live more with less总第2段方法一：encourage young children to donate lessused things分第3段方法二：make it a habit for them；train ourselves第4段结论：find joy in a single object总议论文中的“转折型段落”(重点是看懂转折后的观点)转折型的段落内容一分为二，前半部分是一种观点，而后面的转折之后
73、才是作者想要强调的关键。无论前面你觉得怎样正确，都要记得转折后才是关键。因此阅读这种段落时，把更多的注意力要放在转折后的内容上。这种段落往往有很明显的转折标志词，如but，yet，however，instead等等。因此还是比较好识别的。根据对历年真题的分析，我们发现正确答案往往出现在转折后。因此希望大家重视这样的转折标志词，有详略的来阅读这样的段落。典例(2018北京卷阅读D节选)Preparing Cities for Robot CarsThe possibility of selfdriving robot cars has often seemed like a futurists
74、dream, years away from materializing in the real world.Well, the future is apparently now.The California Department of Motor Vehicles began giving permits in April for companies to test truly selfdriving cars on public roads.The state also cleared the way for companies to sell or rent out selfdrivin
75、g cars, and for companies to operate driverless taxi services.California, it should be noted, isnt leading the way here.Companies have been testing their vehicles in cities across the country.Its hard to predict when driverless cars will be everywhere on our roads.But however long it takes, the technology has the potential to change our transportation systems and our cities, for better or for worse, depending on how the transformation is regulated.行文脉络句叙述了无人驾驶机器人汽车将成为现实。句转折词But是整段甚至是全文的关键，表明了作者的观点：无人驾驶汽车是将会更好还是更坏地改变我们的交通系统、改变我们的城市，这将取决于政策制定者如何规划、管理这项科技。