No one knows exactly this important discovery was made.A. that　　　B. that how 　　C. what 　　

问题补充：

No one knows exactly this important discovery was made.

A. that

B. that how

C. what

D. when

答案：

A. that

B. that how

C. what

(答案→)D. when

解析：考查名词性从句。when什么时候，句意：没有人确切的知道什么时候发现这个重要的发现。