Question

【题目】______ at the door before you enter my room, please.
A.Knock
B.Knocking
C.Knocked
D.To knock

Answer 1

【答案】A
【解析】句意：麻烦您进我屋之前先敲门。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。选A。