A.Economic Principles

　　B.Law of Supply and Demand

　　C.M ore Goods，Lower Prices

　　D.Fewer Goods，Higher Prices

　　该问题，A泛指经济规律，面过宽；C和D只是供需规则的二个单独的方面，不能包含全部，面太窄。正确的答案是B，因为这正是该短文自始至终谈论的主题。

　　三、一段文章的中心思想常常由主题句（Topic Sentence）表达。主题句常常位于段首或段尾处，间或出现在段落中间。同样，一篇文章的中心思想也常常在开始或结尾段点出。因此在阅读中，我们要特别注意文章的开头或结尾。例四：

　　English is clearly an international language.It is spoken by pilots and airport control operators on all the airways of the world.Over 70 percent of the world"s radio programs are in English.

　　本段主题句为首句，其后的内容句均说明首句。例五：

　　Some students prefer a strict teacher who tells them exactly what to do.Others prefer to be left to work on their own.Still others like a democratic discussion type of class.No one teaching method can be devised to satisfy all students at the same time.

　　本段中心思想在结尾句得到体现，它是此段内容的结论。例六：

　　For adults a cold is not that serious.However，this is not the case for children.Cold symptoms in children may be signs of more serious diseases and should be given prompt medical attention.

　　本段主题句为第二句，因为在however后面才是作者真正的看法，并在下文中进一步作了说明。

　　然而，我们也常常发现，有的主题思想并非直接由一、二个句子表述，而是在文中间接暗示着。这就需要读者根据文中细节进行概括与归纳，找出主题思想。例七：

　　It"s no wonder that we have so many rejections for physical reasons in our Selective Service.Since October of 1948，some six million young men have been examined for military duty.Of these，more than a million were rejected as physically unfit for military service.To get two men today，the United States Army must call seven men.Of the five rejected three are turned down for physical reasons and two for mental problems.To get the 196thousand additional men we needed for Berlin，the government had to call up 750 thousand men.And the rejection rate is increasing each year.The reason for this problem is that we are underexercised as a nation.We look，instead of play.We ride，instead of walk.The way we live prevents us from being healthy.

　　本段中找不到某一句作为主题。根据文中的内容进行归纳，可找出主题思想，即：“The military"s rejection rate for physical reason is increasing because this nation"s lack of exercise is leading to poor health.”

　　在确定一篇文章的中心思想时，我们通常用快读的方法，从头至尾将全文浏览一遍，注意不要被个别词、句难住而停下来。要从上下文的连贯意思上来理解全文，看看全文谈论的是什么主题（Subject）、作者是从哪几个方面（Aspects）来进行阐述的、有关这个主题我们了解了些什么。这样，就可以比较有把握地概括出中心思想了。