—Did you meet the professor at the airport?

—No，he ____ by the time I ____ there．

A．had left；got

B．has left；reached

C．was leaving；got

D．left；had got

如果（）变化，并不改变工步的划分。

A.加工表面

B.切削刀具

C.切削用量

D.走刀次数