Question

【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

Answer 1

【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。