Thinking that you know in fact you don’t is a serious mistake.A. what　B. that　C. when　D.

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(答案→)A. what　
B. that　
C. when　
D. however
解析：本题考查学生对主语从句和宾语从句的穿插理解。在that you know      in fact you don’t宾语从句中缺少一个能承前启后的联系；同时又作know和you don’t know的宾语,因此只有A项,what疑问代词最充分。