题目

During the period of recent terrorist activities, people _____ not to touch any unattended bag. A . had always been asked B . are always being asked C . are always asking D . always asked 答案：B如图所示，小球由静止从轨道最高处的A点滚下到最低处的B点时，下列说法中正确的是 [　　] A．小球在A点时只有动能，没有重力势能 B．小球在A点时只有重力势能，没有动能 C．小球从A点到B点的过程中，动能转化为重力势能 D．小球从A点到B点的过程中，机械能增加