Map with large caption that says "Plains" and an area is yellow with headers that read "Skidi," "Pawnee," "Chaui," "Pittahauenat," and "Kitkehaki." The yellow area also features rivers, the North Loop, the Loop, the Middle Loop and the Platte, among others. The green area has headers that say "Omaha," "Ponca," "Yankton," "Oto," and "Kansa." The Missouri River and a number of small tributaries are featured in this region. National Atlas of the United States, May, 2013

Review this map of the Native American tribes of the Great Plains. What is the best explanation as to why there are so many tribes in this area of the United States?

The tribes do not have the ability to cross the rivers.
There is a shortage of uncontested tribal land in this area.
The rivers provide the tribes with abundant food resources.
The very mountainous region creates cultural divisions among the tribes.