Q: You are presented with the quiz "What is a common cause for the development of a lake bed?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "avalley is formed by a river flowing" and "most of the valley floors are old lake beds". So, what's the best answer to the question?

Choices:
[1]. paralysis;
[2]. thunder and lightning;
[3]. flowing river;
[4]. erosion;
[5]. rainfall;
[6]. flooding;
[7]. Sediment;
[8]. fulmination;
A:
[3].