Please knock at the door before e__________ the office.

We knock _____people’s doors, and they usually give us some candy _____ a treat.

A. on, to           B, at, to           C. on, as           D. on, on

We should knock at the door ________．

1. A.
   tightly
2. B.
   suddenly
3. C.
   lightly
4. D.
   fiercely