单项选择题
A.2mg/L B.3mg/L C.4mg/L D.5mg/L
A.0.1mg/L B.0.3mg/L C.0.5mg/L D.0.75mg/L
A.0.5MΩ B.1MΩ C.1.5MΩ D.2MΩ
A.0.1A B.0.5A C.1A D.1.5A
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