We have done things we ought not to have done and undone things we ought to have done.
A. left
B. leave
C. will leave
D. leaving
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(答案→)A. left
B. leave
C. will leave
D. leaving
解析：考查时态。句意“我们把不应该做的事情做了，反而把该做的事情给丢下了。”and后的动词是和前面的done并列的，故用left。故选A。