3.一段中两个相距较远的例子说明该段主题句

　　问题中涉及同一段中两个例子、而且它们不在一处时，通常它们说明该段主题句。例如，

　　Mass transportation revised the social and economic fabric of the American

city in three fundamental ways. It catalyzed physical expansion, it sorted out people and land uses, and it accelerated the inherent instability of urban life. By opening vast areas of unoccupied land for residential expansion, the omnibuses, horse railways, commuter trains, and electric trolleys pulled settled regions outward two to four times more distant from city centers than they were in the pre-modern era. In 1850, for example, the borders of Boston lay scarcely two miles from the old business district; by the turn of the century the radius extended ten miles. Now those who could afford it could live far removed from the old city center and still commute there for work, shopping, and entertainment. The new accessibility of land around the periphery of almost every major city sparked an explosion of real estate development and fueled what we now know as urban sprawl. Between 1890 and 1920, for example, some 250,000 new residential lots were recorded within the borders of Chicago, most of them located in outlying areas. Over the same period, another 550,000 were plotted outside the city limits but within the metropolitan area. Anxious to take advantage of the possibilities of commuting, real estate developers added 800,000 potential building sites to the Chicago region in just thirty years – lots that could have housed five to six million people.

　　18. Why does the author mention both Boston and Chicago?

　　(A) To demonstrate positive and negative effects of growth.

　　(B) To show that mass transit changed many cities.

　　(C) To exemplify cities with and without mass transportation.

　　(D) To contrast their rates of growth

　　问题中的关键词Boston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for example。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：Mass transportation revised the social and economic fabric of the American city in three fundamental ways。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。

　　(本文撰写时参考了李传伟已出版的《TOEFL阅读高分对策》和即将出版的《新托福阅读高分对策》。)

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