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 it doesB.what it doesC.what it isD.why it is
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C解析考查强调句型和宾语从句。本题的主句是it seems to me that,在when引导的时间状语从句中含有what引导的宾语从句,且宾语从句为强调句型。