下列各组物质中，前者是纯净物，后者是单质的是[ ]
A.空气、氧气
B.氯酸钾、氮气
C.粗盐、二氧化碳
D.氢气、氧化镁
答案：B
Atlanta ____ a series of fierce fights between blacks and whites in the 1960s. [ ]
A. met
B. saw
C. happened
D. made
煤气
