Fact 1: Avalley is formed by a river flowing.

Fact 2: Most of the valley floors are old lake beds.

Given the two facts above, answer the question "What is a common cause for the development of a lake bed?" with the following options: 
- paralysis
 - thunder and lightning
 - flowing river
 - erosion
 - rainfall
 - flooding
 - Sediment
 - fulmination


 

   flowing river      