When I try to understand______that prevents so many Americans from being as happy as 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
A. A．why B.what C.what D.why
【参考答案】
C 这里考核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......