at the door before you enter the office or you’ll be considered impoliteA. KnockingB. Knoc

问题补充：

at the door before you enter the office，or you’ll be considered impolite

A. Knocking

B. Knocked

C. Knock

D. To knock

答案：

A. Knocking

B. Knocked

(答案→)C. Knock

D. To knock

解析：考查祈使句。本题考查祈使句 + and / or + 陈述句结构，故用动词原形。句意：在进办公室之前敲门，否则你会被认为不礼貌。故C正确。