A、25%. B、1/3. C、50%. D、40%. C题目询问美国年轻人占总人口的百分比是多少?关键是听到“25岁以下的年轻人约占美国总人口的一半”，可判断选项C正确。

Millions of words have been written about young people in the United States. There are reasons for this great interest in the ideas, feelings, and actions of youth.
    [32] Today there are about seven million Americans in colleges and universities.  [33] Young persons under twenty-five make up nearly half of the American population. [34] Many of these will soon be in charge of the nation. Naturally their ideas are important to everyone in the country, and it is necessary for older people to understand what they think and feel.
    College students today have strong opinions about right and wrong. [35] They are deeply interested in making a better life for all people, especially for those who have not been given a fair chance before now. They see much that is wrong in the lives of their parents. It is hard for them to see what is right and good in the older way. As a result, there is often trouble in American families. Your country may be meeting such problems, too.