An analyst gathered the following information about a stock index: Mean net income for all companies in the index $ 3.2 million Standard deviation of net income for all companies in the index-$ 4.8 million If the analyst takes a sample of 36 companies from the index, the standard error of the sample mean is closest to:（）

A. $ 66667.

B. $ 88889.

C. $ 800000.

目前公用使用哪些酸（），（）、（）、（）。