Q 24 What is a dummy variable? Select one: O a. A variable used in a regression model when the dependent variable is a categorical variable O b. A variable not included in the regression model that does affect the dependent variable. O c. A variable not included in the regression model that does affect the independent variable O d. A variable used in a regression model when the independent variable is a categorical variable. O e. A variable, other than the treatment, that can affect the outcome of the experiment