81 random samples were selected from a normally distributed population and were found to have a mean of 32.1 and a standard deviation of 2.4. Construct a 90% confidence interval for the population standard deviation.
a) 2.1 < σ < 2.8
b) 1.4 < σ < 1.8
c) 28.8 < σ < 37.4
d) 4.6 < σ < 7.8