单选题
When I try to understand ______ that prevents so many Americans from being happy as one might expect, it seems to me that there are two causes.
【正确答案】
B
【答案解析】名词性从句。what引导的名词性从句作动词understand的宾语，宾语从句是一个it is...that...的强调句型，that之后的prevents缺少主语，故正确答案为B。