Final answer:
The price elasticity of demand is zero when the demand curve is vertical.
Step-by-step explanation:
The price elasticity of demand is zero when the demand curve is vertical. This is because a vertical demand curve means that the quantity demanded does not change regardless of the price. In other words, the demand is completely inelastic. When demand is inelastic, the percentage change in quantity demanded is always zero, regardless of the percentage change in price.