Complementary colours are colours that are combined to make a grayscale off colours
A complementary pair is when you combine colours that are opposite of each other like red and green, or yellow and purple etc.
When you put two of these colours together they cancel each other out which leads to that grayscale of colours like grey or even brown
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.
I think its B a beach at low tide
