Additional government purchases equal the supply shift at price P Steps:

• Quantity demanded at P remains constant between years.
• In year 1, government buys excess supply (Qs on S minus Qd), labeled as segment (S S1).
• In year 2, supply shifts right to S1, increasing Qs at P by the horizontal distance between S and S1.
• Additional purchases = year 2 excess (Qs on S1 minus Qd) minus year 1 excess = shift distance between S and S1 at P. Why D is correct:
• YZ measures the horizontal shift between S and S1 at P, matching the extra quantity supplied that government must buy to maintain P. Why the others are wrong:
• A. WZ is total year 1 purchases, not the increase.
• B. XY is part of year 1 excess, unrelated to shift.
• C. XZ is total year 2 purchases, exceeding the additional amount.

Final answer: D