题目内容：
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
参考答案：
选择题：When I try to understand ________ that prevents
- 题目分类：语言/文学
- 题目类型：选择题
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