We all know trees play an important role in ______

试题详情及答案解析

We all know trees play an important role in _______up the air.

A．clean	B．cleaning
C．cleaned	D．Cleans

答案：B

试题分析：句意：我们都知道树在清洁空气中扮演了很重要的角色。介词in后跟动词时要用动词的ing形式，故选B。
考点：考查非谓语动词的用法