镁粉是制造烟花的重要原料之一。生产镁粉时,将镁蒸气在某种气体中冷却。下列可作为冷却气体的是[ ]
A.空气
B.氮气
C.氩气
D.二氧化碳
答案:C
链式网络
I watched ______ last night. [ ]
A. TV about an hour
B. an hour TV
C. an hour for TV
D. TV for an hour