If the price elasticity of demand is infinite, the demand curve is

• A. Vertical
• B. Horizontal
• C. Upward-sloping
• D. Downward-sloping

Answer: (B)

Price elasticity of demand measures how responsive quantity demanded is to price changes. If the elasticity is infinite, buyers react to almost any price change, willing to adjust the quantity demanded without limit at that price. On a graph, this extreme responsiveness is shown as a horizontal line: at the given price, the quantity demanded can be any amount, but if the price rises even slightly, buyers stop buying (demand collapses to zero).

So a horizontal demand curve best represents perfectly elastic demand. The other shapes don’t fit this idea: a vertical curve would mean quantity demanded doesn’t change with price (perfectly inelastic); an upward-sloping curve would imply higher prices increase quantity demanded (unusual and not aligned with the standard demand law); a typical downward-sloping curve reflects ordinary demand but not infinite elasticity.