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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A解析考查动词时态的用法。句意是“我们把不该做的事情做了并且留下了应该做的事情”，句子主干是个并列句，即we have done things we ought not to have done和left undone things we ought to have done，所以have done和have left是并列谓语，其中have相同，省略，只不过两个并列句里分别含有一个定语从句而已，第一个分句中we ought not to have done作things的定语，第二个分句中we ought to have done作things的定语，不要受定语从句的干扰，故选A。本题简单。