He's never been late for class, ______ he? [ ]
A. is
B. isn't
C. has
D. hasn't
答案:A
分别说出四种流量调节方法?
()在一个数据集中出现最多的数值,用来表现集中趋势。
A.算数平均数
B.中位数
C.众数
D.极差