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。