at the door before you enter my room, please.
A. Knock
B. Knocking
C. Knocked
D. To knock
网友回答
(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。