6. McDonald's restaurants do the bulk of their business at lunchtime, but have found that promotionaily- priced meals at breakfast and dinner make a signicant prot contribution. Does the success of McDonald's restaurants in this regard reect an effective application of the marginal prot concept or the incremental prot concept? Explain. Characterize each of the following statements as true or false, and explain your answer. (a) If marginal revenue is less than average revenue, the demand curve will be downward sloping. (b) Prots will be maximized when total revenue equals total cost. (c) Given a downward-sloping demand curve and positive marginal costs, prot- maximizing rms will always sell less output at higher prices than will revenue-maximizing rms. (d) Marginal prot is the difference between marginal revenue and marginal cost and will always equal zero at the prot-maximizing activity level