You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Q: Why are the rocks and pebbles found on riverbeds usually smooth?
(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.
A:
B