题目

Mr. Samson was a newspaper editor in Boston, America. On the night of August 28, 1988, when he was on night duty, he dreamed that a volcano(火山)near the Island of Java(爪哇岛) erupted(爆发) , and that the local people were buried under lava(熔岩).   When he woke up from the bad dream with a start, he thought it was very interesting material, so he wrote it down on a piece paper, left it on the desk and went home. Early in the morning, the editor-in-chief came to work and saw the paper. He took it for the latest news that something important had happened last night and published it i静止在地面上的小物体，在竖直向上的拉力作用下由静止开始运动，在向上运动的过程中，物体的机械能与位移的关系如图所示，其中0--s1过程的图线是曲线，s1--s2过程的图线为平行于横轴的直线．关于物体上升过程（不计空气阻力）的下列说法正确的是 （    ）        A．0--s1过程中物体所受的拉力是变力，且不断减小        B．s1--s2过程中物体做匀速直线运动        C．0--s2过程中物体的动能先增大后减小        D．0--s2过程中物体的加速度先减小再反向增大，最后保持不变且等于重力加速度