A local pizza delivery store varies the amount of online advertising (X1, $100s), radio advertising (X2, $100s), and price promotion (X3 = 1 if price is reduced 20%, 0 if no price reduction). They follow unit sales (Y, pizzas sold) over n = 16 consecutive time periods.

Based on the multiple linear regression model with independent and normally distributed errors of constant variance. For parts 3 and 4, use Model 3.

Fit the following 3 Models: