单选题I just don't understand__

问题补充：

单选题Ijustdon understand_______thatpreventssomanyAmericansfrombeingashappyasonemightexpect.A.why it doesB.what it doesC.what it isD.why it is

答案：

C解析本题是宾语从句中的强调句使用。可以将原句恢复：“It is what that prevents so many Americans…”但是英语中疑问代词或副词需放句首。