问题描述：
He made it in the end.but he didn't explain to us what great difficulty he had had _______ the
problem.
A. working out B.to work out C.work out D worked out.答案是A,我选的是D,为什么,写出理由
problem.
A. working out B.to work out C.work out D worked out.答案是A,我选的是D,为什么,写出理由
问题解答：
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