Unit elastic supply has constant Es = 1 for linear curves through the origin

Steps:

• Price elasticity of supply Es = (%ΔQ supplied / %ΔP) ≈ (dQ/dP) × (P/Q).
• For a linear supply curve, dQ/dP is constant (reciprocal of slope in P-Q space).
• Constant Es requires (dQ/dP) × (P/Q) = constant, so P/Q must be constant.
• P/Q constant implies P proportional to Q, so the curve passes through (0,0).

Why B is correct:

• A line through the origin gives Q = kP, so dQ/dP = k and P/Q = 1/k, yielding Es = k × (1/k) = 1 everywhere.

Why the others are wrong:

• A: Vertical intercept >0 means P = a + bQ (a>0); Es = (1/b)(P/Q) varies, >1 near low Q and approaches 1 at high Q.
• C: Horizontal intercept >0 means P = a + bQ (a<0); Es = (1/b)(P/Q) varies, <1 near the intercept and approaches 1 at high Q.
• D: Vertical line means dQ/dP = 0, so Es = 0 (perfectly inelastic).

Final answer: B