Had they known what was ing next, they second thoughts.
A. may have
B. could have
C. must have had
D. might have had
网友回答
A. may have
B. could have
C. must have had
D. might have had
解析：本句是一个倒装句，可还原为：if they had known what was ing 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，由此可见答案D符合语境。