The teacher apologized          late.

A．to his students to arrive

B．to his students for arriving

C．at his students to arrive

D．at his students for arriving

结合升降式单向阀的功能特点，下面描述正确的一项是（）。

A、该阀门不能装在水平管道上

B、该阀门不能装在竖直管道上

C、可安装于任何位置

D、在阀瓣上部设置辅助弹簧，阀瓣在弹簧的作用力下关闭，则可安装于任何位置