液压式车轮制动器分为（）和（）两大类

从学校到科技馆，王军用10分钟，李虹用8分钟，王军和李虹的速度比是4：5．______．（判断对错）

链传动中链条的节数采用奇数最好。

.Since I came here two years ago, great changes __________ in the factory and many people want to come here to pay a visit.

A have been taken place   B. had been taken place  C. have taken place   D. are taking place

鸡兔同笼，有35个头，94条腿，鸡有______只，兔有______只．