You are presented with the quiz "What do rivers do to rocks?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "soil is formed by rocks eroding" and "rivers erode the rocks they flow over". So, what's the best answer to the question?

Possible answers: 1). storing water 2). to move people 3). they bend 4). Make them get bigger 5). It has no effect on rocks 6). Polish them 7). Turn it to soil 8). protect them
Answer:
7).