插齿加工时，切削的主运动是（ ）。Gear shaping, the main motion of cutting is ( ).
选项：

A:插齿刀的旋转运动 the rotary motion of the gear shaper;
B:插齿刀的上下往复运动 the reciprocating motion of the gear shaper;
C:工件的旋转运动 the rotational motion of the workpiece;
D:插齿刀的径向切入运动 the radial cutting motion of the gear shaper