属于季铵盐类消毒剂的是（）
A.新洁尔灭
B.过氧化氢
C.碘仿
D.漂白粉
E.70%酒精
参考答案：A
The mean and standard deviation of the net incomes for all companies making up a stock index is $ 3.5 million and $ 4. 2 million, respectively. If a random sample of 49 companies is taken from the index, the standard error of the sample mean is closet to:（）
A. $85714.
B. $500000.
C. $600000.
可塑性
