It was ________ we were having dinner at the table ________ there was a knock at the door．

1. A.
   where; while
2. B.
   which; as
3. C.
   when; after
4. D.
   when; that

It was         we were having dinner at the table         there was a knock at the door.

       A．where; while        B．which; as            C．when; after          D．when; that