—What a pity! Their experiment is a failure.
—They better under more favorable conditions.
A. should have done
B. could have done
C. must have done
D. may have done
网友回答
A. should have done
(答案→)B. could have done
C. must have done
D. may have done
解析： 句意：--真遗憾,他们的实验是一个失败.—在更加有利的条件下他们本应该做的更好。这里under more favorable conditions.是一个隐含的虚拟语气,相当于if they had been under more favorable conditions.说明是和过去相反的虚拟条件句,所以主句是could have done“过去本可以做的更好”，所以选B。