20. I got up early this morning but I so because I was not on duty today.A. mustn’t have

问题补充：

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

答案：

A. mustn’t have done

(答案→)B. needn’t have done

C. didn’t need to do

D. can’t have done

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