21.Statistics professor Dr. Stat claimed his pulse rate at the peak of an exercise program was lower than the mean pulse rate of statistics students. Dr. Stat's pulse rate was measured to be 60.0 bpm. His 20 students had a mean pulse rate of 74.4 bpm with a standard deviation of 10.0 bpm. What critical value you should use to test Dr. Stat's claim that statistics students have a mean pulse rate greater than 60.0 bpm at a 0.01 significance level?

a.2.325

b.2.528

c.2.539

d.2.845

e.2.861