—I hear Kris had an accident as he was riding his bike to school yesterday.

—________？ He is always so careful when he is riding.

A．What for

B．How come

C．Why that

D．So what

定轴轮系传动比的数值与其齿数的多少( )。

A．无关
B．有关
C．成正比
D．成反比