A potato farmer was sent to prison just at the time when he______have been digging the gro

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A potato farmer was sent to prison just at the time when he______have been digging the ground for planting the new crop of potatoes.

A.must

B.might

C.could

D.should

请帮忙给出正确答案和分析,谢谢!

热搜:a是b的充分条件,b是a的必要条件吗