A monopolist has set her level of output to maximize profit. The firm's marginal revenue is

$20, and the price elasticity of demand is -3.0. Which of the following prices or price ranges

describe the firm's profit maximizing price:

a. $0-9.

b. $10-19.

c. $20.

d. $21+.

e. This problem cannot be answered without knowing the marginal cost.