单项选择题
A.1/4 B.1/3 C.1/2 D.2/3
A.2m/s;0.1m/s B.2m/s;0.2m/s C.4m/s;0.1m/s D.4m/s;0.2m/s
A.1D B.2D C.3D D.4D
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