置换管道内空气应采用（）或其他无腐蚀、无毒害性的惰性气体作为隔离介质。
参考答案：氮气
-Has your brother found an apartment yet?
-No, he’s still living in a room downtown, but I ________ him look. [ ]
A. help
B. have helped
C. am helping
D. helped
下列处理方法能够净化水的是（ ）
①过滤②吸附③沉淀④蒸馏．
A．①④
B．①②④
C．②③
D．①②③④
