<h2>D) The closing of the frontier</h2><h2 /><h2>Explanation:</h2><h2 />
A time after the Oklahoma Land Rush, the executive of the U.S. Census Bureau declared that the frontier was closed. The 1890 census had shown that a frontier line, a point behind which the population frequency was less than two persons per square mile, no longer survived. This news had a tremendous emotional influence on many Americans.
