Why are the rocks and pebbles found on riverbeds usually smooth?

Pick your answer from:
[1]. The rocks and pebbles in riverbeds are not very old..
[2]. The rocks and pebbles rub against each other as water flows over them..
[3]. Rivers can only flow over smooth rocks and pebbles..
[4]. Organisms in the rivers break down the rocks and pebbles..
[2].