A form of market structure studied by economists is monopoly. When is a firm a monopoly, or are monopolies only theoretical concepts that do not exist? O A. A firm is a monopoly if it earns economic profits at least in the short run. O B. Monopolies do not exist because many markets have barriers to entry. O C. Monopolies do not exist because just about every product has substitutes. O D. A firm is a monopoly if it can ignore other firms' prices. O E. A firm is a monopoly if its economic profits are competed away in the long run