_____ about it earlier, we could have wamed people of the danger

_____ about it earlier, we could have wamed people of the danger.
A、Would we know
B、Should we know
C、Have we known
D、Had we known
【正确答案】：D
【题目解析】：本题考查if引导的虚拟语气从句的倒装。原句应为If we had known about it earlier, 省略if的倒装句应为：Had we known