Final answer:
The demand curve facing a perfectly competitive firm is perfectly elastic and can sell any quantity at the market price, while a monopolist faces a downward-sloping demand curve.
Step-by-step explanation:
The demand curve facing a perfectly competitive firm is perfectly elastic. This means that the firm can sell any quantity it wishes at the prevailing market price. Unlike a monopolist, who is the only firm in the market and faces a downward-sloping demand curve, a perfectly competitive firm can sell its output at the same price regardless of the quantity.