at the door before you enter my room please.A. Knock B. 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：考察句式结构。考生若能识别before是状语从句连接词，便可想到空格位置应该是主句部分的谓语，只有A项符合条件。祈使句考点常混在选项是非谓语动词的题中，常考点还有：祈使句，and/or sb. will…。如Study hard, and you will pass the exam. / Study hard, or you will fail the exam.句意：麻烦您进我屋之前先敲门。