下列指标属于相对比的是（）。
A．极差
B．平均预期寿命
C．变异系数
D．均数
E．方差
参考答案：C
Mr Zhou is very _______ with his students. He asks them not to be late. If they are late, they have to stay outside the classroom until the next class.
A．welcome
B．patient
C．strict
D．happy
如何正确使用套筒？
