Answer: A. The moving river cut through the rock of the valley over time.
Explanation:
Weathering and erosion are responsible for the valley being created. Over time, as the river kept flowing, weathering took place and the rock was broken down and cut through by the river. This process would have happened over millions of years.
After the rocks were broken down, erosion took place as the river then transported the broken down rock to another area thereby creating the valley.
This is the process by which the Colorado river formed the Grand Canyon.
