题目

At school many things happen to us. We may feel excited when we have success in a school play. We may feel sorry if we lose an important game. We want to keep the memory for the rest of our lives. How to keep the memory? Our English teacher, Miss Wang, has taught us a good way of remembering things to make our own yearbook. What is a yearbook? A yearbook is a kind of book which is used to keep the memory of exciting moments. It’s usually made at the end of the year. Last December, we began to make our yearbook. First we chose the persons who had done something special, then some stu如图甲所示，质量不计的弹簧竖直固定在水平地面上，t=0时刻，将一金属小球从弹簧正上方某一高度处由静止释放，小球落到弹簧上压缩弹簧到最低点，然后又被弹簧弹起离开弹簧，上升到一定高度后再下落，如此反复．通过安装在弹簧下端的压力传感器，测出这一过程弹簧弹力F随时间t变化的图象如图乙所示，则（ ）A.t2时刻弹簧的弹性势能最大B.t3时刻弹簧的弹性势能最大C.t1～t3这段时间内，弹簧的弹性势能先减小后增加D.t1～t3这段时间内，弹簧的弹性势能先增加后减少