Because an individual perfectly competitive producer cannot affect the market price of a good, it faces the horizontal demand curve D_C, as shown in panel (a), allowing it to sell as much as it wants at the market price. A monopolist, though, can affect the price. Because it is the sole supplier in the industry, it faces the market demand curve D_M, as shown in panel (b). To sell more output, it must lower the price; by reducing output, it raises the price.