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 causes.
Awhy it does
Bwhat it does
Cwhat it is
Dwhy it is
正确答案
答案解析
该题考查句子结构。根据题干，“that...might expect”在句中做understand的宾语。宾语从句可由连接词that引导，that仅起连接作用，常可省略。若连接词在句中兼起一个句子成分的作用，则应使用what,how,when,why,who,which,where等连词，但要用陈述语气，根据句意答案为C，连接词在句中做表语。