单选题 When I try to understand ______ that prevents so many Americans from being as happy as one might expected, 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] 这个句子比较复杂。It seems to me that there are two causes是主句。when引导的状语从句也是一个复合句，understand后面跟宾语从句，而且这个从句是一个强调句型：It’s what that prevents…。