正态曲线下面积有一定的规律，μ±1σ的面积占总面积的()
A．50.5%
B．55.2%
C．62.8%
D．68.2%
E．69.2%
参考答案：D
Before testing this method on patients, Dr. Baker had tests for two years
on three hundred dogs.
A．found out
B．helped out
C．worked out
D．carried out
离心式压缩机中采用（）效率最高。
A、前弯叶轮
B、后弯叶轮
C、径向叶轮
D、都一样
