单选题 4.  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.
【正确答案】
【答案解析】 本题考查强调句型。强调句型的基本结构是“It is/was+被强调的部分+that+句子其他成分”。此处强调的是句子的主语what，而疑问词应提到句首，故答案为C。