例3
　　For years, nuclear-power advocates have claimed that nuclear power is the most economical form of energy available; but in light of a few facts, one begins to doubt this claim. The cost of building the Sequoiah nuclear plant, for example, exceeded a billion dollars. For this astronomical amount of money, one can expect this reactor to be out of operation approximately 30% of the time. After 30 or 40 years, it will become too “hot” to operate and will be shut down permenantly. Even though the reactor will be shut down, it will still be highly radioactive and will have to be totally encased（包住）in concrete and lead—all at a cost of another few million dollars—and guarded virtually forever. Nuclear power is neither cheap nor economical; it is both expensive and wasteful.
　　Q: Which of the following best summarizes the passage?
　　A) Nuclear power is the most inexpensive form of energy available.
　　B) Although nuclear power costs a lot, it can be used permanently.
　　C) It is not economical to use nuclear power.
　　D) Nuclear power is very popular in many countries.
　　本段共有六句。句提出的只是段落所要议论的主题，即核能的经济效益问题，没有明确地陈述作者对问题的看法。接下来四句以西克沃伊核电站为例，分析其造价高而工作寿命极其有限的弊病。后一句合乎逻辑地引出作者分析这一事例而得出的结论。
　　由此可知，C）项，“使用核能经济上不划算”，为正确答案。A）、B）两项与原文内容不符，D）项内容原文并未提及。
　　与主题句位于开段的情况相反，主题句位于收段时，段落的写作程序是：表述细节的句子在前，概述性的句子居后，并以此收段。通常用归纳法撰写的段落大抵如此。
　　位于收段的主题句往往陈述依据上文的细节推出的结论或建议、归纳的要点或共性，以及得出的观察结果或印象。
　　例4
　　British and American universities are similar in their pursuit of knowledge as a goal but are quite different in their organization and operation. English universities and colleges are relatively small. American universities, which combine a number of different colleges and professional schools, are large, sometimes with 20,000 to 25,000 students on one campus. Teacher training colleges and polytechnics（工艺专科学校）are alternatives to the university course for some students in England, being established for specific purposes.
　　In contrast, virtually all schools of education, engineering and business studies, are integral （完整的）parts of universities in the United States. In England, universities receive about 70% of their financial support through Parliamentary （国会的） grants. Similarly, in the
　　United States, public institutions receive about 75% of their funds from local, state, and federal sources, but private colleges and universities receive little or no government support.
　　In England, personal finacial aid is provided by the government to over 80% of the students, through local education authorities, according to the parents’ income. In the U.S., student aid is given by the university or the sponsoring agency and is provided by private organizations and the state or federal governments. Obviously, British and American universities have similar educational aims but different means for achieving these aims.
　　Q: From the passage we learn that________.
　　A) the goal of education between British and American universities are quite different
　　B) the ways of running the universities are quite different between England and America
　　C) the British universities include a lot of colleges
　　D) the American universities include a lot of organizations
　　本文主要通过对照来说明英国大学与美国大学的不同点。句除指出它们都是以传授知识作为目标这一共同点之外，主要概述它们的不同点，即在学校的组织和管理上有很大的区别。为说明这一看法（即段落的主题思想），作者随即陈述若干细节，包括学校的规模、教学组织结构、办学经费的来源以及学生助学金的来源与管理等四个方面。后，作者使用逻辑承接词obviously，既总结上述细节，又引出一个结论性的句子作为收段。显然，首尾两句表述的是同一个主题思想，只是句型结构和用词略有不同。
　　从段落中可知，“英、美两国大学的管理方法有所不同”，因而选B）。这句话实际上概括了段落大意。其余几个选项均不合原意。
　　一般来说，一个段落只围绕着一个主题来表述主旨大意，只是有些段落在结构上比较严谨，采用了前呼后应，两次点题，从而更加突出主题思想的写法。两个主题句表述的是同一个主题，即后者重述前者所表述的思想内容。但两个主题句在句型结构和用词方面往往不尽一致，而且在内容上后者也不是前者的简单重复，多半有所引申或顺应其间细节的铺叙而另有侧重。
　　例5
　　The average population density of the world is 47 persons per square mile. Continental densities range from no permanent inhabitants in Antarctica to 211 per square mile in Europe.
　　In the western hemisphere, population densities range from about 4 per square mile in Canada to 675 per square mile in Puerto Rico. In Europe the range is from 4 per square mile in Iceland to 831 per square mile in the Netherlands. Within countries there are wide variations of population densities. For example, in Egypt, the average is 55 persons per square mile, but 1,300 persons inhabit each square mile in settled portions where the land is arable（可耕的）.
　　Q: The best title for the passage would be________.
　　A) Great Differences in Population Density of the World
　　B) The Average Population Density of the World.
　　C) Different Populations in Different Countries.
　　D) People and the World.
　　句“世界人口的平均密度为每平方英里47人”不是主题句，因为后续的句子都不是用来支撑它所表述的意思。该句仅起引介主题的作用，表明下文要论述的是 population density。据此可以确定本段论述的主要对象是人口密度，而且后续细节也都紧扣这一主题，如大陆的、西半球的、欧洲的以及一国之内的人口密度。再从所有细节涉及的共同点来看，都论及“人口密度的差距”这一方面。
　　据此，A）项，“世界人口密度的巨大区别”，为该文的标题。B）、C）两项：“世界人口的平均密度”、“不同国家的不同人口”，都没有准确点到题。至于D）项， “人民和国家”，则太笼统，也不切题。
　　没有主题句的段落，如上一段，主要通过对细节的陈述含蓄地表现段落的主题思想。即使段落中有的句子表述的内容具有概括性，但不是演绎的前提，也不是归纳的结果，仍属细节或仅起引介主题的作用。
　　对于无主题句的段落，不可能采用辩认主题句的方法来获取段落的主题思想。这里，宜采用酝酿段落题要或标题的方法，把不同的细节所集中论述的要点概括出来。
　　尽管不一定达到主题句那样完整，但只要切合段落的主题，即可表达该段的主题思想。
　　使用这种方法，要求读者在正确理解所有细节的基础上，发挥自己的逻辑概括能力。
　　3.1.2 辩认主题展开的重要细节(Recognizing Primary Supporting Details)
　　细节是组成文章的主体部分，是用来阐明主旨大意的，或者说，是用来支撑主题思想的。要注意识别句子的功能及文中的承接语，以便充分理解文章的主旨及具体细节和事实。