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______ was unknown to us, he always said, was how the teacher knew the secret.

A. It  
B. What
C. That  
D. As
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科目：高中英語 來源：同步題 題型：單選題

________ was unknown to us, he always said, was how the teacher knew the secret.
[     ]
A．It  
B．What
C．That  
D．As

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