单选题When I try to unders

问题补充：

单选题WhenItrytounderstand_______thatpreventssomanyAmericansfrombeingashappyasonemightexpect,itseemstomethattherearetwocauses.A.why it doesB.what it doesC.what it isD.why it is

答案：

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。