Why were the mound builders mound builders

Mound Builders, in North American archaeology, name given to those people who built mounds in a large area from the Great Lakes to the Gulf of Mexico and from the Mississippi River to the Appalachian Mts. The greatest concentrations of mounds are found in the Mississippi and Ohio valleys. The term Mound Builders arose when the origin of the monuments was considered mysterious, most European Americans assuming that the Native Americans were too uncivilized for this accomplishment. (Hoped this help)

It's most likely choice a, it makes the most sense in my opinion.

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