- Water causes soil and rock erosion
- Water flows down a slope
- Hence, water flowing down slopes causes soil erosion

- Water does not easily flow on a flat surface
- Terraces change slopes into a series of flat steps
- So, terraces change slopes into a kind of surface that water does not easily flow over

- Water flowing down slopes causes soil erosion
- Terraces change slopes into a kind of surface that water does not easily flow over
- So, terraces decrease / reduce soil erosion on a slope

- Terraces decrease / reduce soil erosion on a slope
- Preventing something decreases the amount of that something
- Hence, terraces prevent soil erosion on a slope

- A mountain / hill is a kind of environment
- A slope is a part of a mountain / hill
- So, a slope is a kind of environment

- A slope is a kind of environment
- Building terraces increases the amount of terraces in an environment
- Hence, building terraces on a slope increases the amount of terraces on that slope

- Terraces prevent soil erosion on a slope
- Building terraces on a slope increases the amount of terraces on that slope
- So, building terraces on a slope prevents soil erosion on that slope

So, building terraces on a slope prevents soil erosion on that slope
[Question]Soil erosion can be best prevented by
[Context]- Preventing something decreases the amount of that something 
- Water does not easily flow on a flat surface 
- Water causes soil and rock erosion 
- A mountain / hill is a kind of environment 
- A slope is a part of a mountain / hill 
- Building terraces increases the amount of terraces in an environment 
- Water flows down a slope 
- Terraces change slopes into a series of flat steps