单项选择题
A.10~15mg/LB.20~25mg/LC.15~20mg/LD.25~30mg/L
A.8.5~10mg/LB.10~12.5mg/LC.12.5~15mg/LD.15~17.5mg/L
A.7~7.2B.7.2~7.4C.7.4~7.6D.7.6~7.8