斜管沉淀池具有( )等优点，但存在斜管费用较高，而且需要定期更新等问题。

A．沉淀效率低

B．节省占地

C．沉淀效率高

D．停留时间短

Don't go to open the door at once when the door bell _____.You should ask who it is at first.       [ ]

A. run off    

B. set off    

C. go off      

D. put off