A physical therapist wishes to estimate, with 99% confidence, the mean maximum strength of a certain muscle from a group of individuals. He assumes that muscle strength valuesare approximately normally distributed with s = 12. Take a sample of 15 subjects and obtain a mean x= 84.3 What is the interval?

a) 84.3 7.993

b) 84.3 9.223

c) 84.3 2.30

d) 84.3 2.662