Use the maps below to answer the following question:

Image of two maps, side by side. The map on the left shows a map of North America showing the regions of Native American cultures. The Southwest culture could be found in the Southwestern United States and in the country that borders the United States to the south. Along the east coast of the United States to the Appalachian Mountain range you will find two cultures the Northeast cultures and the Southeast cultures. The map on the right shows physical features in North America. The Rocky Mountains run from Canada in the north and south through the United States. A smaller mountain range is in the East. The Appalachian Mountains run along the East Coast of the United States. The center of the United States is mainly flat.

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Compare the two maps. Which geographical feature formed the western boundary of the Plains Native American cultural region? (4 points)

a
The Appalachian Mountains

b
The Rocky Mountains

c
The Mississippi River

d
The Pacific Ocean