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.
A. why does it
B. what does it
C. what it is
D. why it is
网友回答
A. why does it
B. what does it
(答案→)C. what it is
D. why it is
解析：句意：当我试图去理解是什么阻挡了许多美国人得到他们应有的快乐时，我发现个两个原因。这是强调句型，其结构是：It was+被强调部分+that/who（人）+其他。判断强调句型的方法是把It was that这三个词去掉，整个句子不缺成分，意思完整。这里被强调的是疑问词what，因为是宾语从句，所以强调句型应该用陈述语序，what在从句中做主语，why不能做主语，故选C。