I just don't understand that prevents so many Americans from
being as happy as one might expect.
A. why it does
B. what it does
C. what it is
D. why it is
网友回答
A. why it does
B. what it does
(答案→)C. what it is
D. why it is
解析：本题是宾语从句中的强调句使用。可以将原句恢复：“It is what that prevents so many Americans…”但是英语中疑问代词或副词需放句首。