－Have you seen Mr. Black these days, Jack?

－Yes, I met him when I ________ in the street yesterday.

A．walked

B．was walking

C．have walked

D．is walking

沿齿轮任意圆周上，轮齿两侧渐开线间的弧长称为（）。

A.齿距

B.齿厚

C.齿宽

D.全齿高