Answer & Explanation: Holding all else constant, a decrease in the market demand for a product in a competitive market would cause the marginal revenue (MR) curve of the firms to shift to the left or downwards. Marginal revenue describes graphically, the additional income generated from the sale of one additional unit of a product or service. Thus, it shows the relation between the marginal revenue received by a firm for selling its product or service and the quantity of product or service that was sold. To be plotted, a comparison is made between the total revenue generated from any given number of sales and the total revenue generated from selling one extra unit.