at the door before you enter my room please.A. Knock B. Knocking C. Knocked D. 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。祈使句都是把动词原形放在句首的。故A正确。句意：进来之前请敲门。故A正确。