[Regression] PSG conducted a regression andysis of shampoo sales on their national, local, and digital advertising expenditures. Response variable is shampoo sales in million dollars, and explanatory variables are the ad spends in million dollars. The dataset is observed during offseason. Thanksgiving. and Christmas. Below are the estimated coefficients. (a) Interpret the meaning of three advertising coefficients (national advertising. local advertising and digital advertising) (10 pts) (b) Interpret the meaning of two control variables iThanksgiving. Christmas) ( 5pts ) (c) Explain the role of control variables. What happens if we omit the control variables in running a regression analysis? (10 pts)