A. To demonstrate positive and negative effects of growth. B. To exemplify cities with and without mass transportation. C. To show mass transportation changed many cities. D. To contrast their rate of growt 参考答案:C 答案解析:推理题。文中第一段第四句指出“举例说,1850年,波士顿市界离老的商业地区几乎不到2英里,到了这世纪末,其半径扩至10英里。现在供得起的人们可以住得很远,远离老城市中心,仍然来回去那里上班、购物和娱乐