Answer:
Throughout the late 1940's and 1950's, MGM became the king of the movie musical. And this was largely due to the "Freed Unit." Freed's productions became so popular that the studio could afford to have a SECOND unit for musicals under the direction of Joe Pasternak.
The Mound Builders used metal tools, and were probably metal builders. Based on that information I would say the answer is B.Copper, because copper is a metal.
