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 different cultural regions. In the south there is Mesoamerica. Above Mesoamerica is the Southwest. Along the Pacific Coast is the Pacific Northwest. Beside the Pacific Northwest and in the center of what is now the United States there is the Great Plains, Great Basin, and Plateau. On the eastern side of the continent is the Eastern Woodlands. In the North, there is the Subarctic and the Arctic. The map on the right shows a map with legend and text: Rocky Mountains, Great Plains, Great Lakes, Appalachian Mountains, Mississippi River, Mountains, Grass & Pacific Ocean, Atlantic Ocean, and Gulf of Mexico.

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

the Appalachian Mountains
the Rocky Mountains
the Mississippi River
the Pacific Ocean