单选题 When I try to understand ______ that prevents so many Americans from being as happy as one might expect, it seems to me that there are two causes.

A. why it does
B. what it does
C. what it is
D. why it is

【正确答案】 C

【答案解析】[解析] 本题考查的是句式比较复杂的强调句。A、B两个选项不符合强调句的基本结构，可首先排除。题中understand的宾语是一个what从句，而从句本身又是一个强调句型，等于it is(what)that prevents so many Americans：from being as happy as one might expect。