Answer:
Perfectly competitive firm: demand curve is a line parallel to the horizontal axis (perfectly elastic demand)
Monopoly: demand curve is a downward sloping line (lower price higher quantity demanded)
Marginal revenue for perfectly competitive firm is constant, marginal revenue for monopoly is decreasing
Step-by-step explanation:
A perfectly competitive firm is a price-taker. It is a tiny player that cannot influence price whether it produces lower or higher quantity. Therefore its revenue increase the same amount (charge the same given price) for every additional unit => constant marginal revenue.
A monopoly is the only firm in its market, its demand curve is the aggregate demand curve, which is downward sloping. For every additional unit it has to charge a lower price (for the added unit and all previous unit) to sell => decreasing margnial revenue.