To make our city more beautiful, rubbish            into the river. [ ]

A. needn't be thrown

B. mustn't be thrown

C. can't throw

D. may not throw

适用于运行速度在2.5m/s以下电梯的减速器为（）。

A．蜗杆蜗轮式减速器

B．双蜗杆减速器

C．直齿轮式减速器

D．无齿轮减速器