Suppose you are the manager in charge of the sales of laptop computers for a national chain located in various cities. Suppose for each city you want to predict sales (in $1000s) based upon (X) advertising expenditure (in $1000s). Based upon a random sample of cities, you obtained the prediction equation Y = 156.4 + 13.08(X1). Which of the following statements is true?

a. You expect to have $156,400 in sales if you spend nothing on advertising.
b. If you spend $0 on advertising, you expect sales of $13,080.
c. If you increase your advertising expenditure by $1000, you would expect an increase in sales of $156,400.
d. All of these statements are true.