单项选择题 A weighted average of the value of a random variable, where the probability function provides weights is known as（）

单项选择题 A sample of 400 observations will be taken from an infinite population. The population proportion equals 0.8. The probability that the sample proportion will be greater than 0.83 is（）

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（）