6. (26%) A monopoly with the constant marginal cost c = 200 offers the product to three groups of consumers with the demand functions D,(p) = 800 - p, D,(p) = 700 -p, and Dy(p) = 300 -p, respectively, where D(p) 20, i= 1,2,3. Find the optimal price and quantity for each group if: (a) the firm is able to price-discriminate different groups; (b) the firm is not able to price-discriminate different groups