The owner of Original Italian Pizza restaurant chain would like to predict sales of his specialty, deep-dish pizza. He has collected data on monthly sales of deep-dish pizzas at his restaurants and observations on other potentially relevant variables for each of his 15 outlets in central Pennsylvania. 

a. Estimate the multiple regression model between quantity sold (Y) and following explanatory variables: average price of deep-dish pizzas, monthly advertising expenditures, and disposable income per household in the areas surrounding outlets.

b. Which of variables in this model have regression coefficients which are statistically different from 0 at the 5% significance level?

c. Given your findings in part c, which variables, if any, would you select to remove from model estimated in part a? Describe your decision.