The Average Natural Gas Bill for a Random Sample of 21

Multiple Choice

The average natural gas bill for a random sample of 21 homes in the 19808 zip code during the month of March was $311.90 with a sample standard deviation of $51.60. The 90% confidence interval around this sample mean is ------.

A) ($266.82, $356.98)
B) ($292.46, $331.34)
C) ($278.09, $345.71)
D) ($285.98, $337.82)