质量为M的物体，以2g的加速度大小竖直向上减速运动距离H，下列说法正确的是（ ）
A．物体的重力势能增加2MgH
B．物体的动能减少2MgH
C．物体的机械能增加MgH
D．物体的机械能减小MgH
A、物体上升的高度是H，重力做负功，重力势能增加，增加的重力势能为MgH，故A错误；
B、由牛顿第二定律可得：F合=Ma=2Mg，由动能定理可得：△EK=-2MgH，则物体的动能减少2MgH，故B正确；
C、在运动过程中，重力做的功为-MgH，则除重力之外，其它力的功为-2MgH-（-MgH）=-MgH，则物体机械能减少了MgH，故C错误，D正确；
故选BD．
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A．to
B．in
C．at
D．on