The grass grew and kept the soil intact. The fence kept the soil contained. The soil was completely gone. The amount of rain decreased.
I gave my students this multiple choice question: Several horses grazed in a fenced area across from a home. On rainy days, soil would wash down a slope and run toward the home. After the horses were moved a few years later, the soil no longer washed down when it rained. What could account for this change?

Only one answer is correct among these 4 choices:
- The grass grew and kept the soil intact.
- The fence kept the soil contained.
- The soil was completely gone.
- The amount of rain decreased.

Could you tell me which one is correct?

The grass grew and kept the soil intact.