题目

It was the first time Mary _____ such a big opening ceremony. A. has attended              B. attended         C. had attended              D. should attend 答案：C下列说法中正确的是（ ）A．物体在恒力作用下不可能做曲线运动B．曲线运动一定是变速运动C．物体在变力作用下不可能做直线运动D．物体做曲线运动的加速度可能为零