A sample of 80 bolts of carpet showed that their average length was 160 yards. The standard deviation of the population is 0-6 yards. using this sample data, which of the following is the 99% confidence interval for the mean length per bolt of carpet?
a. (158.44, 161.56)
b. (155.85, 164.15)
c. (157.93, 162.07)
d. (158.27, 161.73)