锯削薄板时，在锯削截面上至少应有( )锯齿同时参加锯削。
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锉削运动时，右手小臂应与锉削面的( )方向，保持基本平行。
A. 左右
B. 上下
C. 前后
Pigeons have been used as messengers because ______.
A. they have a built-in compass
B. they can cover 500 miles in a single day
C. they have special ability to tell directions
D. they can find their way home
Why is the problem of drinking and driving difficult to solve
An alcohol is easily obtained.
B. Drinking is linked to organized crime
C. Legal prohibition has already failed
D. Legislation alone is not sufficient
刃磨錾子时，切削刃要与錾子几何中心线（）。
A. 倾斜
B. 平行
C. 垂直
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