If a demand curve has a zero slope (horizontal line), demand is A. inelastiC. B. unit elastiC. C. perfectly inelastiC. D. perfectly elastiC. E. elastiC.

The correct answer is D. perfectly elastic.<br /><br />When a demand curve has a zero slope (horizontal line), it means that the quantity demanded is perfectly elastic. This implies that consumers are willing to purchase any quantity of the good at a specific price, and even a small change in price will result in an infinite change in quantity demanded. In this case, the demand is perfectly elastic.