In the Southwestern U.S., tectonic forces have pushed rocks upward over millions of years to form the Colorado Plateau. As the plateau has risen during this time, the Colorado River has been cutting down through the plateau to form the Grand Canyon. Development of the Grand Canyon through the uplift of the plateau and erosion by the Colorado River illustrates that

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I believe the answer to this is D because in order for the Grand Canyon to form i needed erosion meaning it needed something to break the rocks down and to form. It took approximately 17 million years. So i think D is the answer but I could be wrong.    

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