A demand curve parallel to the Y-axis indicates

Analyze the question: The question asks what kind of price elasticity of demand is shown by a demand curve that runs parallel to the Y-axis (the vertical axis).

Concept: In a demand diagram, price (P) is on the Y-axis and quantity (Q) is on the X-axis. Price elasticity of demand (PED) is:

PED = % change in quantity demanded ÷ % change in price

Working through it:

• A line parallel to the Y-axis is a vertical line.
• On a vertical line, quantity stays the same at every price. For example, the quantity demanded is 100 units whether the price is ₦50 or ₦500.
• So the % change in quantity demanded = 0, no matter how much the price changes.
• Therefore PED = 0 ÷ (% change in price) = 0.

A PED of zero means demand is perfectly inelastic. Consumers do not change the quantity they buy at all when the price changes. Examples are life-saving drugs such as insulin for a diabetic patient.

Checking the other options:

• B. Perfectly elastic demand is a horizontal line (parallel to the X-axis), with PED = infinity.
• A and D (fairly elastic and fairly inelastic) are downward-sloping curves, not straight vertical lines.
• C. Perfectly inelastic is the vertical line, which is what the question describes.

Memory tip: Think of the letter I in "Inelastic". It is a straight, upright line, like a vertical line. Elastic perfectly (∞) is flat like the "E" arms, horizontal. Another way to remember is "Vertical = Very stubborn": the quantity won't move whatever the price does.