题目

The private car has long played an important role in the United States. In 1971 eighty—three percent of American families owned at least one car, and twenty--three percent had more than one. By giving workers rapid, convenient transportation, the car has freed them from having to live near their place of work. This has helped develop the growth of the suburbs, but it has also led to traffic problems in the city. In addition, the car has caused neighborhood ties to be weak by making it easy to keep up friendship at a distance and to enjoy leisure activities far from home. For farm families th如图所示是质量为1kg的滑块在水平面上做直线运动的v-t图象，下列判断正确的是（　　）A．在t=1s时，滑块的加速度为零B．在1s～5s时间内，合力做功的平均功率为2WC．在4s～6s时间内，滑块的平均速度大小为2.5m/sD．在5s～6s时间内，滑块受到的合力大小为2N