The estimated multiple regression equation for a model with two independent variables (x1 and x2) and 15 observations follows: = 35-63 + 16x1 + 20X2 Interpret bi (coefficient for x1) in this estimated regression equation. a. bi indicates a 1-unit change in y for a 16-unit change in xl when X2 is held constant. b. 01 indicates a 1-unit change in y for a 16-unit change in xi when X2 is allowed to vary c. 01 indicates a 16-unit change in y for a 1-unit change in X1 when X2 is held constant. d. by indicates a 16-unit change in y for a 1-unit change in x1 when X2 is allowed to vary