单项选择题
A.≤0.2dB B.≤0.25dB C.≤0.5dB D.≤0.8dB
A.≥8dB B.≤6dB C.≥3dB D.≤5dB
A.10s B.20s C.30s D.60s
A.1.2 B.1.5 C.2 D.1.5-2
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