为确保干熄焦系统安全，循环系统中配备的充氮点有（）。

A.循环风机前充氮点

B.循环气体回流充氮点

C.循环风机后充氮点

D.空气导入充氮点

“Did you hear someone knocking at the door?”  

“Yes, I did. I heard him______three times.” [ ]

A. knocking      

B. knocked      

C. being knocking  

D. knock