单项选择题
A.30mg/m3(20ppm) B.20mg/m3(30ppm) C.3mg/m3(2ppm) D.2mg/m3(3ppm)
A.一级 B.二级 C.三级 D.四级
A.30mg/m3 B.30ppm C.100mg/m3(150ppm) D.150mg/m3(100ppm)