______ at the door before entering please.

______ at the door before entering please.

A．Knocked          B．Knock            C．Knocking          D．To knock

_____ at the door before entering please.

A. Knocked 　 B. To knock　 C. Knocking 　 D. Knock

______ at the door before entering please.

1. A.
   Knocked
2. B.
   Knock
3. C.
   Knocking
4. D.
   To knock

A．Knocked

B．To knock

C．Knocking

D．Knock