Never such a difficult problem. It took me almost half an hour to work it out.A. had I co

问题补充：

Never such a difficult problem. It took me almost half an hour to work it out.

A. had I come across

B. I had come across

C. did I come across

D. I did come across

答案：

A. had I come across

B. I had come across

C. did I come across

D. I did come across

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