2.设地球质量为M,半径为R,万有引力恒量为G,则-|||-一块质量为m的无动力火箭残骸,当它从距地面h1-|||-高处下降到h2高处时,若不考虑空气阻力,火箭残-|||-骸增加的动能应等于() ()-|||-A dfrac ({h)_(2)-(h)_(2)}((R+{h)_(1))(R+(h)_(2))}-|||-B dfrac ({h)_(1)-(h)_(2)}({R)^2}-|||-C dfrac ({h)_(2)-(h)_(1)}((R+{h)_(1))(R+(h)_(2))}-|||-D dfrac ({h)_(2)-(h)_(1)}({R)^2}