______about it earlier,we could have warned people of the danger.

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问题 ______about it earlier,we could have warned people of the danger.

选项 A、Would we know
B、Should we know
C、Have we known
D、Had we known

答案D

解析句意：如果我们早点知道的话，就会警告人们有危险发生。本句为虚拟语气结构，主句为could have warned引导的现在完成时态，从句要用过去完成时态had done的形式。在if省略的条件下，虚拟条件句中的从句部分要将助动词had放到主语之前，进行倒装。因此本题选D。

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