Assertion (A): At every point on a straight-line supply curve that pa…

Assertion (A): At every point on a straight-line supply curve that passes through the origin, the price elasticity of supply is equal to 1.

Reason (R): The price elasticity of supply does not depend on the units in which price and quantity are measured.

Answer & explanation

Correct answer: option 2

The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).

Both statements are true. A follows from the geometric method; R is a general property of elasticity and does not show why the value is 1.

Assertion (A): At every point on a straight-line supply curve that passes through the origin, the price elasticity of supply is equal to 1. — True. By the geometric method, e_S at a point S is Mq₀/Oq₀. For a line through the origin, M lies at the origin, so Mq₀ = Oq₀ and e_S = 1.

Reason (R): The price elasticity of supply does not depend on the units in which price and quantity are measured. — True. Elasticity is a ratio of two percentage changes, so it is independent of units.

Why R does not explain A?

R only says that the measure is free of units. It holds for every supply curve, whatever its elasticity, so it cannot tell us why this curve gives a value of 1.

The real reason for A is the position of the line. When the line starts from the origin, the distance Mq₀ becomes the same as Oq₀ at every point, so the ratio is 1.

NCERT (2026-27): “4.7 Price Elasticity of Supply … When the supply curve is vertical, supply is completely insensitive to price and the elasticity of supply is zero. In other cases, when supply curve is positively sloped, with a rise in price, supply rises and hence, the elasticity of supply is positive. Like the price elasticity of demand, the price elasticity of supply is also independent of units.”

“The Geometric Method … Now we come to panel (b). Here the supply curve goes through the origin. One can imagine that the point M has coincided with the origin here, i.e., Mq0 has become equal to Oq0. The price elasticity of this supply curve at the point S is given by the ratio, Oq0/Oq0 which is equal to 1. At any point on a straight line, supply curve going through the origin price elasticity will be one.”