WAEC Economics 2017 Paper

Concept:A demand curve parallel to the $Y$-axis means quantity demanded does not change when price changes.Explanation:On a demand curve graph, the $Y$-axis shows price and the $X$-axis shows quantity demanded.When the demand curve is parallel to the $Y$-axis, it is a vertical straight line.At every price level, the quantity demanded remains constant at $Q = Q_0$.If price rises from $P_1$ to $P_2$, quantity demanded still stays $Q_0$.So, the percentage change in quantity demanded is zero.Price elasticity of demand is:$$E_d = \frac{\%\ \text{change in quantity demanded}}{\%\ \text{change in price}} = \frac{0}{\text{positive number}} = 0.$$An elasticity value of $0$ means demand is perfectly inelastic.Answer:C. perfectly inelastic