Observe the following figure : In the above figure, the elasticity of supply at A on the linear supply…

By the geometric formula for point elasticity of supply on a straight-line supply curve whose extension meets the horizontal quantity axis at $B$, the price at point $A$ is $AQ$, the quantity is $OQ$, and the reciprocal slope $\frac{dQ}{dP}$ is $\frac{BQ}{AQ}$. Therefore, $e_s = \frac{dQ}{dP} \cdot \frac{P}{Q} = \frac{BQ}{AQ} \cdot \frac{AQ}{OQ} = \frac{BQ}{OQ}$, matching "BQ / OQ".