题目

It was difficult to tell exactly _______ the saying began, but it is possible that it was in the theatre or movie industry. A．where                           B．when                   C．why                           D．that  答案：A弹簧振子做简谐运动时，以下说法正确的是（    ）A．振子通过平衡位置时，弹力一定为零B．振子做减速运动时，加速度可能在减小C．振子向平衡位置运动时，加速度方向与速度方向相反D．振子远离平衡位置运动时，回复力方向与速度方向相反