单项选择题
A.3倍 B.2倍 C.1.5倍 D.1.3倍
A.1~5ms B.5~10ms C.10~20ms D.5~30ms
A.20~25m B.15~20m C.10~15m D.5~10m
A.25dB B.20dB C.15dB D.10dB
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