Additional government purchases equal the increase in supply at support price P

Steps:

• At price P, quantity demanded Qd is identical both years along demand curve D.
• Year 1 supply S1 intersects P at quantity Qs1; government buys surplus Qs1 - Qd.
• Year 2 supply S2 (shifted right) intersects P at larger quantity Qs2; government buys Qs2 - Qd.
• Extra purchases in year 2: (Qs2 - Qd) - (Qs1 - Qd) = Qs2 - Qs1, the horizontal distance between S1 and S2 at P.

Why D is correct:

• YZ measures Qs2 - Qs1, the exact increase in surplus due to expanded supply at fixed price P.

Why the others are wrong:

• A. WZ: Likely total year 1 surplus (Qs1 - Qd), not the difference.
• B. XY: Possibly a vertical price segment or unrelated quantity, ignoring supply shift.
• C. XZ: May represent total year 2 quantity supplied, not the incremental purchase.

Final answer: D