题目

如图所示，轻杆的一端有一个小球，另一端有光滑的固定轴O．现给球一初速度，使球和杆一起绕O轴在竖直面内转动，不计空气阻力，用F表示球到达最高点时杆对小球的作用力，则F A．一定是拉力          B．一定是推力 C．一定等于0           D．可能是拉力，可能是推力，也可能等于0 答案： D Hardly ______ she heard the strange scream coming from behind a tree.A.had she walked out of the woods thanB.did she walk out of the woods whenC.had she walked out of the woods whenD.she had walked out of the woods when