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
(答案→)D. might have had
解析：句意：要是他们早知道接下来所发生的事，他们可能已经改变想法了。Had they known ...是对过去情况的虚拟，相当于If they had known ...根据过去情况虚拟语气的构成，主句用 would/should/could/might+have+过去分词，可知此空要填：might have had 。