He made it in the end.but he didn't explain to us what great difficulty he had had _______ theproblem.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

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

have difficulty (in) doing something
He didn't explain to us he had had great difficulty (in) working out the problem.

D。很明显，过去完成时态，第一个had加强语气的。