车轮踏面是车轮在钢轨面上（）接触的部位，具有一定的斜度，踏面与轨面在一定的摩擦力作用下完成车轮的滚动运行。
A.滚动
B.滑动
C.蛇行运动
参考答案：A
She _________ it, but that she saw it with her own eyes. [ ]
A. couldn't have believed
B. couldn't believe
C. can't believe
D. didn't believe
财产共有关系中的共有人只能是自然人。( )
