What Made the American West Such an Exceptional Type of "Settler

What made the American West such an exceptional type of "settler society"?

A) It included migrants from overseas who had traveled thousands of miles by ship.
B) It included groups of natives with existing cultures and histories of their own.
C) It built on a mostly arid landscape with a variety of geographic and climatic extremes.
D) It brought together not just two or three, but multiple different immigrants and natives.