The monthly earnings of teachers is normally distributed with a mean of $3,000 and the standard deviation of $250. We select a sample of 87 teachers. The sampling distribution of the sample mean has an expected value and standard deviation of:
a.) 3,000 and 26.8
b.) 3,000 and 1.69
c.) 3,000 and 250
d.) 3,000 and 2.87
e.) 3,000 and 321.6