单项选择题

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

A.a probability function
B.a random variable
C.thee xpected value
D.random function

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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（）

A.0.4332
B.0.9332
C.0.0668
D.0.5668
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（）

A.1.1022
B.2
C.30
D.1.4847
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