Question

I was doing my homework last night ________ I heard a cry ________ help．

1. A.
   then; of
2. B.
   while; in
3. C.
   at the same time; to
4. D.
   when; for

Answer 1

D
本句中的when连接两个分句，前分句用过去进行时，表示某个动作正在进行，突然……。