20. I got up early this morning, but I so because I was not on duty today.
A. mustn’t have done
B. needn’t have done
C. didn’t need to do
D. can’t have done
网友回答
A. mustn’t have done
(答案→)B. needn’t have done
C. didn’t need to do
D. can’t have done
解析：可联系答.案.网客服索取。