But for their mon interests, they cooperated so smoothly in the investigation of the case.
A. could have
B. will not have
C. could not have
D. have not
网友回答
A. could have
B. will not have
(答案→)C. could not have
D. have not
解析：考查隐含的虚拟语气：句意：要不是他们的共同兴趣，他们就不会在这个投资项目上合作的这么顺利。But for是“要不是”，是隐含的虚拟条件句，But for their mon interests=if it hadn’t been for their mon interests，所以主句也是虚拟语气，和过去的情况相反用would/could/should have done.选C