The main aim of a perfectly competitive firm is to maximize economic profit. In a perfectly competitive market, each firm can sell what quantity they want to sell at the market equilibrium price. A firm`s demand curve is perfectly elastic at the equilibrium market price. This means that at any price higher than the market equilibrium price, a firm is not able to sell anything at all, as buyers are well-informed and can obtain the identical product for a lower price without bearing extra search cost. One firm`s product is a perfect substitute for any other firm’s product. The correct answer is E.