at the door before entering please. A. Knocked B. KnockC. Knocking D. To knock

问题补充：

at the door before entering please.

A. Knocked

B. Knock

C. Knocking

D. To knock

答案：

A. Knocked

(答案→)B. Knock

C. Knocking

D. To knock

解析：句意:在进去之前请敲一下门。这里是祈使句，这个句子缺少谓语动词，前面没有主语，所以只能用动词原形开头，故选B。