单项选择题
A.3.6mg/L B.6.9mg/L C.8.7mg/L D.7.3mg/L
A.一级 B.二级 C.三级 D.二级、三级均可
A.0.75mg/m3 B.0.50mg/m3 C.0.45mg/m3 D.0.60mg/m3