单项选择题
A.小于60dB B.大于等于60dB C.大于等于65dB D.大于70dB
A.10%；5% B.5%；7% C.15%；7% D.2%；5%
A.10dBμV B.6dBμV C.3dBμV D.0dBμV
A.10 B.50 C.100 D.1000
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