at the door before you enter my room please.A. KnockB. KnockingC. KnockedD. To knock

问题补充：

at the door before you enter my room, please.

A. Knock

B. Knocking

C. Knocked

D. To knock

答案：

(答案→)A. Knock

B. Knocking

C. Knocked

D. To knock

解析：考查祈使句。祈使句句首应该使用动词原形。