A

斯下面是题目：
1)
A town has 500 real estate agents. The mean value of the properties sold in a year by these agents is $ 800,000, and the standard deviation is $ 300,000. A random sample of 100 agents is selected, and the value of the properties they sold in a year is recorded.
a)
What is the standard error of the sample mean?
b)
What is the probability that the sample mean exceeds $ 825,000?
c)
What is the probability that the sample mean exceeds $ 780,000?
d)
What is the probability that the sample mean is between $ 790,000 and $ 820,000?