A company is interested in comparing the mean sales revenue per salesperson at two different locations. The manager takes a random sample of 10 salespersons from each location independently and records the sales revenue generated by each person during the last 4 weeks. He decides to use a t-test to compare the mean sales revenue at the two locations. Which of the following assumptions is necessary for the validity of the t-test?
(A) The population standard deviation at both locations are equal.
(B) The population standard deviations at both locations are not equal.
(C) The population standard deviations at both locations are known.
(D) The population of the sales records at each location is normally distributed.
(E) The population of the difference in sales records computed by pairing one salesperson from each location is normally distributed.