下列说法正确的是(     )    A.空气的流动不属于机械运动  

下列说法正确的是(    

   A.空气的流动不属于机械运动        B.只有机器的运动才是机械运动

   C.参照物必须是绝对不动的物体       D.任何物体如果以自身为参照物,它的速度就是零

答案

D

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