Final answer:
For a horizontal demand curve, the slope is 0 and the price elasticity of demand is undefined.
Step-by-step explanation:
The slope of a demand curve represents the rate of change in quantity demanded relative to the change in price. For a horizontal demand curve, the slope is 0 because the quantity demanded remains constant regardless of price changes. The price elasticity of demand measures the responsiveness of quantity demanded to changes in price. For a horizontal demand curve, the price elasticity of demand is undefined because the percentage change in quantity demanded is infinite when the percentage change in price is 0.