Giving reasons, distinguish between the behaviour of demand curves of firms under perfect competition and monopolistic competition.
Demand curve of the firm under perfect competition: The demand curve is perfectly elastic. It means that a firm can sell any amount of the commodity at the prevailing price. Even a fractional rise in price would wipe out the entire demand for the firm's product. Firm's demand curve is indicated by a horizontal straight line parallel to X - axis.
Demand Curve of the Firm under Monopolistic Competition
Under monopolistic competition, the firm faces a negatively sloped demand curve. It means that a large quantity of the commodity can be sold only by decreasing its price. The demand curve is also more elastic because of availability of close substitutes in the market.