A simple random sample of size n = 21 is drawn from a population that is normally distributed. The sample mean is found to be x = 56, and the sample standard deviation is found to be s = 13. Construct a 95% confidence interval about the population mean. Round to two decimal places.
a) Lower bound: 51.12, Upper bound: 60.88
b) Lower bound: 50.50, Upper bound: 61.50
c) Lower bound: 52.31, Upper bound: 59.69
d) Lower bound: 53.44, Upper bound: 58.56