问答题 按加工原理不同，齿轮齿形加工可以分为哪两大类？各有何优缺点？

单项选择题 引起传导性耳聋最常见的原因是( ) A．过敏 B．耵聍 C．中耳炎 D．胆脂瘤 E．先天性闭锁

单项选择题 —We took a taxi and arrived at the station an hour earlier. —You ______ a taxi. Time was enough for you to go there by bus. A．neednt take B．neednt to take C．neednt have taken D．dont need take