Question

Mike! Can you hear someone ________ at the door?

1. A.
   knock
2. B.
   to knock
3. C.
   knocking
4. D.
   is knocking

Answer 1

C
hear sb. knock at the door听见某人敲门了；hear sb. knocking at the door听见某人在敲门。根据本句的语境判断。