At any point where a monopolist's marginal revenue is positive, the downward- sloping straight-line demand curve is a. perfectly elastic. b. elastic, but not perfectly elastic. c. unit elastic. d. inelastic.

Marginal revenue is the change in total revenue from selling one more unit of a product. Elasticity of demand refers to the percentage change in quantity demanded in response to a percentage change in price. It comes in different forms: 1. Perfectly elastic demand: a small change in price leads to an infinite change in quantity demanded. 2. Elastic demand: a small change in price leads to a larger change in quantity demanded (elasticity > 1). 3. Unit elastic demand: a small change in price leads to exactly equal change in quantity demanded (elasticity = 1). 4. Inelastic demand: a small change in price leads to a smaller change in quantity demanded (elasticity < 1).

In the case of a downward-sloping straight-line demand curve, the relationship between marginal revenue, the price P, and elasticity E is given by the formula: \[MR = P \left(1 - \frac{1}{E}\right)\] We need to find the elasticity when the marginal revenue is positive.

a. Perfectly elastic demand: In this case, the elasticity E would be infinite. From the formula, the Marginal revenue MR equals Price P times 0 because (1 - 1/E) is equal to 0. Therefore, MR = 0, which does not meet our criterion of a positive marginal revenue. b. Elastic demand: Here, the elasticity is greater than 1 which means (1 - 1/E) is a positive value. Consequently, the Marginal Revenue is equal to the Price times a positive value, which leads to a positive Marginal Revenue. This option satisfies our criterion of a positive marginal revenue. c. Unit elastic demand: Here, the elasticity is equal to 1. Therefore, (1 - 1/E) = 0. As we found in the perfectly elastic case, this leads to a marginal revenue of 0, which does not satisfy our criterion. d. Inelastic demand: In this case, the elasticity is less than 1. Therefore, (1 - 1/E) becomes negative. This will lead to a negative Marginal Revenue, which does not satisfy our criterion. Thus, based on our evaluation: The correct answer is b. Elastic, but not perfectly elastic.