A significant difference between monopoly and perfect competition is that:

A. free entry and exit is possible in a monopolized industry but impossible in a competitive industry.

B. competitive firms control market supply but monopolies do not.

C. the monopolist's demand curve is the industry demand curve while the competitive firm's demand curve is perfectly elastic.

D. profits are driven to zero in a monopolized industry but may be positive in a competitive industry.

I actually don’t know

The correct answer is C. In a monopoly, there is only one seller or producer in the market, while in perfect competition, there are many sellers. This leads to a significant difference in the demand curves faced by the monopolist and the competitive firms.

In a monopoly, the demand curve faced by the monopolist is the industry demand curve. This is because the monopolist is the sole producer and has control over the market. The monopolist can set the price at which it wants to sell its products, and the quantity demanded will depend on that price. As a result, the monopolist's demand curve is downward sloping.

On the other hand, in perfect competition, each firm faces a perfectly elastic demand curve. This means that the firm can sell as much as it wants at the prevailing market price, without having any influence on the price itself. This is because in a perfectly competitive market, there are so many sellers that no individual firm has the power to affect the price.

The other options are not correct because:

A. Free entry and exit is possible in both monopolized and competitive industries. In a monopolized industry, entry may be limited due to the barriers to entry created by the monopolist, but it is still possible.

B. Competitive firms in perfect competition also have control over market supply. Each individual firm in a perfectly competitive market has a very small market share, but collectively, they determine the market supply.

D. Profits can be positive in both monopolized and competitive industries. In a monopolized industry, the monopolist can earn positive profits in the long run if it can maintain its market power and prevent new entrants. In a competitive industry, firms can also earn positive profits if they can differentiate their products or have a cost advantage. However, in the long run, economic profits are driven to zero in perfect competition due to the entry and exit of firms.