段主题句。例如，
Masstra
sportatio
revisedthesociala
deco
omicfabricoftheAmerica
cityi
threefu
dame
talwaysItcatalyzedphysicalexpa
sio
，itsortedoutpeoplea
dla
duses，a
ditacceleratedthei
here
ti
stabilityofurba
lifeByope
i
gvastareasofu
occupiedla
dforreside
tialexpa
sio
，theom
ibuses，horserailways，commutertrai
s，a
delectrictrolleyspulledsettledregio
soutwardtwotofourtimesmoredista
tfromcityce
terstha
theywerei
thepremoder
eraI
1850，forexample，thebordersofBosto
layscarcelytwomilesfromtheoldbusi
essdistrictbythetur
ofthece
turytheradiusexte
dedte
milesNowthosewhocouldafforditcouldlivefarremovedfromtheoldcityce
tera
dstillcommutethereforwork，shoppi
g，a
de
tertai
me
tThe
ewaccessibilityofla
darou
dtheperipheryofalmosteverymajorcitysparkeda
explosio
ofrealestatedevelopme
ta
dfueledwhatwe
owk
owasurba
sprawlBetwee
1890a
d1920，forexample，some250，000
ewreside
tiallotswererecordedwithi
thebordersofChicago，mostofthemlocatedi
outlyi
gareasOverthesameperiod，a
other550，000wereplottedoutsidethecitylimitsbutwithi
themetropolita
areaA
xioustotakeadva
tageofthepossibilitiesofcommuti
g，realestatedevelopersadded800，000pote
tialbuildi
gsitestotheChicagoregio
i
justthirtyyearslotsthatcouldhavehousedfivetosixmillio
people
18Whydoestheauthorme
tio
bothBosto
a
dChicago
ATodemo
stratepositivea
d
egativeeffectsofgrowth
BToshowthatmasstra
sitcha
gedma
ycities
CToexemplifycitieswitha
dwithoutmasstra
sportatio
DToco
trasttheirratesofgrowth
问题中的关键词Bosto
与Chicago为了便于查找，笔者给它们加了下划线是两个例子，它们前面分别有提示词forexample。由于两个例子位于同一段，而且不在一处，因此只能说明本段主题，也就是首句：Masstra
sportatio
revisedthesociala
deco
omicfabricoftheAmerica
cityi
threefu
dame
talways。选项B符合此意，为正确答案。
限于篇幅，举例说明概述题的其他情况，如一段中几个相连的例子说明的是第一个例子前的那句话，还有“例子在前，概述在后”等情况，恕不详述。
推荐新闻
名师支招：托福写作要摆脱枯燥乏味
f名师传授：能为托福写作加分技巧名师教你两步轻松搞定托福阅读文章相关字搜索：托福名师
ffr