To help the sick people in the hospital is _____ for me .

A．good experience

B．a good experience

C．good experiences

D．well experiences

如图是固定在水平桌面上的圆弧轨道，将一小球由轨道的P点自由释放，滚动到Q点．在此过程中，动能先变______后变______，重力势能______．