筛板塔的突出优点有()。
A．结构简单
B．造价低廉
C．板上液面落差较小
D．操作弹性范围较窄
参考答案：A,B,C
--- You've left the light on.
--- Oh , so I have. _____and turn it off.
A．I've gone
B．I'll go
C．I go
D．I'm going
