The manager of a grocery store has taken a random sample of 100 customers. The average length of time it took these 100 customers to check out was 3 minutes. It is known that the standard deviation of the population of checkout times is 1 minute. The 95% confidence interval for the true average checkout time (in minutes) is
a. 1.36 to 4.64.
b. 2.5 to 3.5.
c. 2.804 to 3.196.
d. 1 to 5.