water causes soil and rock erosion & water flows down a slope ->  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 ->  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 ->  terraces decrease / reduce soil erosion on a slope  terraces decrease / reduce soil erosion on a slope & preventing something decreases the amount of that something ->  terraces prevent soil erosion on a slope  a mountain / hill is a kind of environment & a slope is a part of a mountain / hill ->  a slope is a kind of environment  a slope is a kind of environment & building terraces increases the amount of terraces in an environment ->  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 -> building terraces on a slope prevents soil erosion on that slope