What factors led to the “closing” of the last frontier?

A year after the Oklahoma Land Rush, the director of the U.S. Census Bureau announced that the frontier was closed. The 1890 census had shown that a frontier line, a point beyond which the population density was less than two persons per square mile, no longer existed.

Explanation:

d is the correct answer

~courtney

this is not a question its simply asking you to read and write a few sentences and   the question on the bottom its fairly easy, you can do this!