Please choose the correct answer to the question: Why are the rocks and pebbles found on riverbeds usually smooth?

Answer1: The rocks and pebbles in riverbeds are not very old. Answer2: The rocks and pebbles rub against each other as water flows over them. Answer3: Rivers can only flow over smooth rocks and pebbles. Answer4: Organisms in the rivers break down the rocks and pebbles.

Answer format: answer1/answer2/answer3/answer4