单项选择题 From a population of 500 elements，a sample of 225 elements is selected.It is known that the variance of the population is 900.The standard error of the mean is approximately（）

单项选择题 Given the following information：Standar ddeviation=8 Coefficient of variation=64% The mean would then be（）

问答题 在显著性水平α＝0.05时能否认为方案A和方案B在药品中转时间上存在差异？