Pony students will feel pain in the soles of their feet when they walk barefoot on cobblestone mountain roads. This is because () A. The contact area between the sole of the foot and the ground increases. The pony walks barefoot on the cobblestone mountain road, the ground pressure is equal to the pony's gravity, and the forces between objects are interactive. The pressure on the pony's foot is equal to the pressure on the ground. The gravity of the pony remains unchanged, and the pressure on the pony's feet remains unchanged. When the foot steps on the cobblestone, the stress area decreases, the effect of pressure on the foot increases, and the sole of the foot feels pain.

So choose D.