What is the underlying geologic explanation for why western South America is so different from eastern South America?

A) The western side is a plate boundary and the eastern side is not.
B) The eastern side is a plate boundary and the western side is not.
C) The western side marks a plate divergence and the eastern side marks a plate convergence.

The western side of South America is along the boundary of the South American Plate and the Nazca Plate, where the latter is subducting beneath the former, leading to the Andes mountains and significant volcanic activity. The eastern side of South America is not along a major plate boundary, which contributes to its geological differences from the west.