People have come to understand prevents so many Americans from being as happy as one migh

People have come to understand prevents so many Americans from being as happy as one might expect.
A. that it is why
B. what does it that
C. why it does that
D. what it is that

A. that it is whyB. what does it thatC. why it does that(答案→)D. what it is that解析：考查强调句型。本题考查的强调句型的特殊疑问句转换而成的宾语从句，任何一个宾语从句都使用陈述语序。本题强调的句子是主语,然后对主语进行提问。句意：人们逐渐地知道究竟是什么阻止如此多的美国人不如人们所预料的那么开心。故D正确。

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