When he came in, I happened my experiment in the lab.A. to do B. to be doingC. to have done D. to be

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编辑：chaxungu时间：2026-04-27 02:43:39分类：高中英语题库

【题目】When he came in, I happened __________ my experiment in the lab.

A. to do B. to be doing

C. to have done D. to be done

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【答案】B

【解析】句意：当他进来时，我刚好正在实验室做实验。happen后跟动词不定式，根据句意可知动词不定式的动作正在进行，且与谓语动词happen同时发生，故用动词不定式的进行式。