题目

 Can you tell me ________?A.    what are they doing that for   B.why they are doing for C.   why are they doing that           D. what they are doing that for 答案：D自然界中有很多物体做曲线运动，在所有的曲线运动中，物体的运动速度（  ）A．方向一定改变B．方向一定不变C．大小一定改变 D．大小一定不变