The rocks and pebbles in riverbeds are not very old. The rocks and pebbles rub against each other as water flows over them. Rivers can only flow over smooth rocks and pebbles. Organisms in the rivers break down the rocks and pebbles.
I gave my students this multiple choice question: Why are the rocks and pebbles found on riverbeds usually smooth?

Only one answer is correct among these 4 choices:
- The rocks and pebbles in riverbeds are not very old.
- The rocks and pebbles rub against each other as water flows over them.
- Rivers can only flow over smooth rocks and pebbles.
- Organisms in the rivers break down the rocks and pebbles.

Could you tell me which one is correct?

The rocks and pebbles rub against each other as water flows over them.