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

I was doing my homework last night, _________ I heard a cry for help．

[　　]

A．when　　　B．then　　　C．which　　　D．while