What activity ended with the closing of the 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.

C, because he has a direct link and profit from the trade. the fastest way by boat to transport those goods is via the panama canal.