Quiz answer: d.

Frost heave occurs when the ground expands as it freezes, lifting the soil and any objects within it. With each subsequent freeze, those objects—rocks, nails, even old farm tools—are brought closer to the surface, and eventually they break through. If you keep finding debris in an otherwise tidy paddock in winter, it may once have been a dumping ground. The safest option is to keep horses off such areas. If that’s not feasible, be extra vigilant about checking for hazards during the winter.