When I try to understand that prevents so many children from being happy, there seem to b

When I try to understand that prevents so many children from being happy, there seem to be three causes.
A. why it is
B. what it does
C. why it does
D. what it is

A. why it isB. what it does C. why it does(答案→)D. what it is解析：句意：当我试图理解是什么阻止这么多的孩子快乐，似乎有三个原因。这里使用了强调句的特殊问句，因为that后面缺少主语，所以用疑问词what，又因为这个强调句的特殊问句是做understand的宾语从句，所以语序要变成陈述句语序。所以选D。

谢谢了

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