A retail store manager believes that extending shopping hours will greatly increase sales. Previous studies, however, indicate that the increase in revenue might not be sufficient to support the rise in operating expenses due to longer working hours (such as additional employee labor charges). The manager gathers data on 25 random days recording the hours of operation along with the sales that day. Keeping his opening hour the same, he closes the store at several different hours during these 25 days. What type of hypothesis test would best help him see if there is a relationship between the hours of operation and sales?
a. One mean (sigma known)
b. One mean (sigma unknown)
c. Mean of differences, using dependent samples (paired data).
d. Difference of two means (independent samples)