要增加车刀的热伸长量，以下措施不可行的是：（）
A.提高切削用量
B.增加刀杆横截面尺寸
C.增加刀片厚度
D.是用高强度加工材料
参考答案：B
28. The farm as well as its neighboring hills we once spent so much time ____ on a new look as recently as last year.
A．on having taken
B．has taken
C．on has taken
D．having taken
在X光管中，阴极到阳极的电子运动速度取决于（）。
A.靶材料的原子序数
B.管电压
C.管电流
D.灯丝电流
