单项选择题
A.5mg/m3(8ppm) B.10mg/m3(9ppm) C.15mg/m3(10ppm) D.20mg/m3(11ppm)
A.20mg/m2 B.30mg/m3 C.40mg/m3 D.50mg/m4
A.4类 B.5类 C.6类 D.7类