题目

B     A mosquito is an insect that is found all over the world．There are thousands of different kinds of mosquitoes in many different sizes and colors．The female mosquito needs blood from vertebrates （脊椎动物）to lay eggs and produce more mosquitoes．She has a special part of her mouth that she uses to suck blood，and her saliva（spit）thins the blood so she can drink it．In fact．It’s the mosquito’s saliva that makes the bites itch（痒）!     A person who gets bitten by a mosquito will notice a wheal，which is a bump around the bite． The wheal will be round如图所示，质量不计的弹簧竖直固定在水平面上，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，小球接触弹簧并将弹簧压缩至最低点（形变在弹性限度内），然后又被弹起离开弹簧，上升到一定高度后又下落，如此反复．通过安装在弹簧下端的压力传感器，测出该过程中弹簧弹力F随时间t变化的图象如图所示，则（　　）A．运动过程中小球的机械能守恒B．t2时刻小球的加速度为零C．t1～t2这段时间内，小球的动能在逐渐减小D．t2～t3这段时间内，小球的动能与重力势能之和在增加