单项选择题
A.15 B.20 C.10 D.30
A.≤15mg/L B.≤40mg/L C.≤20mg/L D.≤30mg/L
A.≥9.5mg/L B.≥7.5mg/L C.≤9.5mg/L D.≤7.5mg/L
A.8 B.5～10 C.10 D.6～9
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