In a sample of 125 hip surgeries of a certain type, the average surgery time was 137.3 minutes with a standard deviation of 22.8 minutes. Construct an 80% confidence interval for the true mean surgery time.
a) (134.7, 139.9)
b) (136.0, 138.6)
c) (135.5, 139.1)
d) (135.8, 138.8)