问题

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

A:Knocking
B:Knocked
C:Knock
D:To knock

正确答案是：

同类题型

下列文学常识的表述，不正确的一项是（）

I lost sight of Wang Ping when we reached _________looked like a large market

Mother warned ________ the electric lamp.

_______ go to the cinema together with me—- Good idea!

will be a twenty-minute break duringthe performance, ______ people can go to the restroom or buy some snacks.