车床车削螺纹试验，产生螺距误差的原因是机床存在（）误差。
A、主轴
B、导轨
C、导轨位置精度
D、传动链
参考答案：D
.__________having hundreds of online "friends" is great, our real and closest connections still lie with those who are a part of our daily lives.
A．As
B．Once
C．While
D．When
车轮踏面需要做成一定的斜度，踏面呈锥形，其作用是？
