单项选择题
A.3mg/m3(2ppm) B.30mg/m3(20ppm) C.5.4mg/m3(2ppm) D.2mg/m3(5.4ppm)
A.一级 B.二级 C.三级 D.四级
A.阈限值 B.安全临界浓度 C.危险临界浓度 D.30mg/m³(20ppm)