A supply curve parallel to the X-axis indicates

1. Analyze the question We need to identify what kind of supply elasticity is shown by a supply curve that is a horizontal line, parallel to the X-axis (the quantity axis).

2. Explain the concept Price elasticity of supply (PES) measures how responsive quantity supplied is to a change in price:

PES = % change in quantity supplied ÷ % change in price

• Horizontal supply curve (parallel to the X-axis): the price is fixed at one level, and suppliers will supply any quantity at that price. A tiny change in price would change the quantity supplied by an unlimited amount. So PES = ∞, which is perfectly elastic (infinitely elastic) supply.
• Vertical supply curve (parallel to the Y-axis): the quantity is fixed no matter the price, so PES = 0, which is perfectly inelastic supply.

3. Solution

• The X-axis shows quantity and the Y-axis shows price.
• A curve parallel to the X-axis is horizontal, which means the price stays constant while quantity can take any value.
• Quantity supplied responds without limit to price changes, so elasticity is infinite.

Checking the options:

• A. Fairly elastic: PES is greater than 1 but finite, so this is a normal upward-sloping curve. ✗
• B. Infinitely elastic: matches the horizontal line. ✓
• C. Fairly inelastic: PES is less than 1. ✗
• D. Perfectly inelastic: this is the vertical line. ✗

4. Answer The correct option is B, infinitely elastic supply.

5. Exam tip Remember: "Horizontal = Highly (infinitely) elastic; Vertical = Valueless response (zero elasticity)." Also note that the same logic applies to demand curves: a horizontal line means perfectly elastic and a vertical line means perfectly inelastic.