We have done thing we ought not to have done and___undone things we ought to have done.A.left B.leaving.为什么是A,B为什么不可以

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We have done thing we ought not to have done and___undone things we ought to have done.
A.left
B.leaving.为什么是A,B为什么不可以

and 前面已经是过去时了...