At Matthews Car Repair, customer demand for a certain brand of motor oil is normally distributed with a mean of 15 gallons and a standard deviation of 6 work-days. To be 95% sure that Compact Car Repair will not run out of oil, we should reorder when motor oil in stock is less than _______ gallons.

a. 24.9

b. 22.6

c. 23.7

d. 20