单选题 When I try to understand_____ that prevent so many Americans from being as happy as one might expect, it seems to me that there are two causes.无

【正确答案】 C

【答案解析】句意：当我试图弄明白是什么阻止了如此多的美国人像人们期望的那样快乐的时候，在我看来，有两个原因。从句中缺少主语，故应选C。