问题补充：
单选题Hadtheyknownwhatwascomingnext,they___________secondthoughts.A.may haveB.could haveC.must have hadD.might have had
答案：
D解析本题考查虚拟语气的用法。本句是一个倒装句，可还原为：if they had known what was coming next, they might have had second thought。根据had known提示，可知是与过去事实相反，因此虚拟语气主句中应该用would/might have done，由此可见
