Knowing , as a matter of fact, one shouldn’t know is a terrible thing．A. whetherB. thatC.

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A. whether
B. that
(答案→)C. what
D. when
解析：考查宾语从句，what为连接词作one shouldn’t know 的宾语，as a matter of fact,为插入语。句意：事实上一个人知道了不该知道的事情是非常可怕的。