The sodium content of a popular sports drink is listed as 225 mg in a 32-oz bottle. Analysis of 15 bottles indicates a sample mean of 233.0 mg with a sample standard deviation of 25.0 mg. What is the standard deviation of the sample mean?

a) 5.50 mg
b) 6.45 mg
c) 7.21 mg
d) 8.33 mg