After Studying a Region in the Northwestern Part of the United

After studying a region in the Northwestern part of the United States, observations and laboratory studies show that a mountain belt juxtaposed to a low-lying area are compositionally and structurally different and hundreds of thousands of years apart in age from each other. Which statement best explains this anomaly?

A) Foreign mountain ranges are accreted onto continental shields bringing unrelated rock types next to each other
B) Having a mountain belt that is compositionally and structurally unrelated to its neighboring low land is not possible
C) Mountain belts are different from the surrounding low-lying areas by the down drop of continental rock under lower mantle rock
D) both (a) and (c)