In an effort to estimate the mean dollars spent per visit by customers of a food store, the manager has selected a random sample of 100 cash register receipts. The mean of these was $45.67 with a sample standard deviation equal to $12.30. Assuming that he wants to develop a 90% confidence interval estimate, the upper limit of the confidence interval estimate is:

a) About $47.69

b) About $2.02

c) Approx. $65.90

d) None of the above