Getting to other planets or to the moon _____ many problems. [ ]

a. involve

b. involves

c. involving

d. to involve

齿轮传动的传动比取决于( )。

A．主动轮转速

B．主动轮与被动轮齿数之比

C．被动轮与主动轮齿顶圆直径之比

D．被动轮与主动轮轴间距离