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。一、问题形式
举例说明概述题，也叫例证题，被ETS称为“修辞目的题”(rhetorical purpose question)，基本形式如下：
The author uses X as an example of…
Why does the author mention X?
The author discusses X in paragraph 2 in order to…
二、解题技巧
举例的目的在于反映概述，所以这类题的一般解题技巧是：绕过例子看概述，对概述进行同义替换的就是正确答案。具体技巧如下：
1.例子说明概述
举例说明概述题经常对应原文的举例for example。做题时可以先根据问题中的关键词确定例子的位置，再阅读例子前的那个句子，这个句子一般就是例子所说明的概述，也就是答案对应的地方。例如，
…sensitivity to physical laws is thus an important consideration for the maker of applied-art objects. It is often taken for granted that this is also true for the maker of fine-art objects. This assumption misses a significant difference between the two disciplines. Fine-art objects are not constrained by the laws of physics in the same way that applied-art objects are. Because their primary purpose is not functional, they are only limited in terms of the materials used to make them. Sculptures must, for example, be stable, which requires an understanding of the properties of mass, weight distribution, and stress. Paintings must have rigid stretchers so that the canvas will be taut, and the paint must not deteriorate, crack, or discolor. These are problems that must be overcome by the artist because they tend to intrude upon his or her conception of the work. For example, in the early Italian Renaissance, bronze statues of horses with a raised foreleg usually had a cannonball under that hoof. This was done because the cannonball was needed to support the weight of the leg…”
Why does the author discuss the bronze statues of horses created by artists in the early Italian Renaissance ?
·To provide an example of a problem related to the laws of physics that a fine artist must overcome
·To argue that fine artists are unconcerned with the laws of physics
·To contrast the relative sophistication of modern artists in solving problems related to the laws of physics
·To note an exceptional piece of art constructed without the aid of technology
根据问题中的关键词bronze statues of horses定位段末，这里有标志词for example，说明它是对前面所说的内容进行例证。前一句These are problems that must be overcome by the artist…就是概述：艺术家必须克服这些问题，因为这些问题破坏他们对艺术作品的构思。这里所说的问题就是上文谈论的物理规律对美术的限制，如必须考虑制作材料的物理特性。而本题对应的原文以例子说明这一点：由于铜的特性，马抬起的前腿下必须有支撑物。综上所述，例子说明它前面的概述，第一个选项符合此意，为正确答案。
