Air resistance reduces acceleration over time

Steps:

• Observe velocity-time graph for stone in air: velocity increases but at decreasing rate.
• Acceleration is slope of velocity-time graph; decreasing slope means decreasing acceleration.
• In air, drag force opposes motion and grows with speed, reducing net force.
• Deduce acceleration decreases due to increasing air resistance.

Why B is correct:

• Newton's second law: net acceleration a = (mg - drag)/m; drag ∝ v or v² increases with time, so a decreases.

Why the others are wrong:

• A: Terminal velocity requires zero acceleration; graph shows ongoing increase.
• C: Not enough information on distances or times in air vs. water.
• D: Constant acceleration would mean straight-line velocity graph; it's curved.

Final answer: B