Frederick Jackson Turner's thesis is best explained in which of the following?

A. America must continue to make war on the Plains Indians and drive them onto reservations

B. It is inevitable that the American frontier will disappear and farming communities out West will be transformed into massive cities

C. The West is an insignificant part of America's development

D. Westward expansion is what shaped the American identity and once the frontier closes Americans may no longer be the people they once were