以齿轮变速之立式钻床钻削中主轴突然停止可能原因为（）。
A.进刀太慢
B.转速太快
C.变速杆定位不正确
D.切削剂太多
参考答案：C
引起测向误差的原因有（）
A.操作误差
B.测向系统误差
C.噪声引起的误差
D.电波传播误差
Environmental scientists are concerned that if industrial pollution was to continue without effective measures, the Earth’s surface would warm up to a dangerous degree in the next few centuries.
