According to the substitution effect of labor supply, when the wage rate goes up: A. the opportunity cost of enjoying leisure goes down. B. it becomes less costly to consume leisure, so people will work more. C. it becomes more costly to consume leisure, so people will work more. D. firms will hire more workers since people are more willing to work. Monopolistically competitive firms differentiate their products by: A. selling products with slightly different physical characteristics. B. creating a special aura or image for the product with advertising. C. selling products at different locations. D. all of the above