问题
单项选择题
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
答案
参考答案：C
解析：
此处是强调句结构it is (what) that prevents so many Americans from being as happy as one might expect...