变异系数是反映变异程度大小的指标，它属于
A．率B．动态数列C．相对比D．构成比E．集中趋势指标
参考答案：C
A large number of engineers ___ to Africa by our government to help the people there every year.[ ]
A. have sent
B. will send
C. are sent
D. was sent
如何延长柱塞泵填料的使用寿命？
