单选题 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 reasons.
【正确答案】 C
【答案解析】[解析] 当我试图弄明白到底是什么原因使很多美国人并不像人们期望的那样幸福时，似乎有两个原因可以解释。
[解析] 考查it is…that强调句型。本题understand后面的从句作宾语，由what引导，同时，what在强调句中作prevents的逻辑主语。由以上分析可见，选项C为正确答案。