I had not been reading for half an hour _____ I heard the steps outside.

I had not been reading for half an hour _____ I heard the steps outside.

1. A.
   when
2. B.
   while
3. C.
   as
4. D.
   since

I had not been reading for half an hour _____ I heard the steps outside.

    A. when                     B. while              C. as                          D. since

I had not been reading for half an hour _____ I heard the steps outside.