单项选择题
A.30m B.40m C.50m D.60m
A.0.5倍 B.0.8倍 C.1.1倍 D.1.5倍
A.0.5m/s B.0.6m/s C.0.63m/s D.0.65m/s
A.10 B.15 C.20 D.30
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