题目

        When I was a boy every holiday that I had seemed perfect. My parents took me by train or by car to a hotel by the sea. All day, I seem to remember. I played on the sands with strange exciting children. We make sandcastles with huge yellow walls, and watched the incoming tide destroy them; we splashed each other in the water and shrieked with excitement. When the tide went out, we climbed over the slippery rocks and stared down at the fish and the seaweed in the rock-pools. In those far-off days the sun seemed to shine constantly and the water was always warm. Sometimes we left t在空中匀速水平飞行的飞机，在投放救灾物资的过程中     A．飞机的重力势能不变，动能减小　　　 B．飞机的重力势能减小，动能不变     C．飞机的重力势能减小，动能减小　　　   D．飞机的重力势能增加，动能减小