Here's a problem to solve: Why are the rocks and pebbles found on riverbeds usually smooth?

Among the 4 following options, which is the correct answer?

- A: The rocks and pebbles in riverbeds are not very old.
 
- B: The rocks and pebbles rub against each other as water flows over them.
 
- C: Rivers can only flow over smooth rocks and pebbles.
 
- D: Organisms in the rivers break down the rocks and pebbles.
 