单项选择题
A.0.5KM B.1KM C.2KM D.3KM
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
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