Which event most contributed to this phenomenon described in this passage?
We are now in a position to see clearly some of the factors
involved in the Western problem. For nearly three centuries
the dominant fact in American life has been expansion.
With the settlement of the Pacific coast and the
occupation of the free lands, this movement has come to a
check. ... [T]he restless, rushing wave of settlement has
broken with a shock against the arid plains. The free lands
are gone, the continent is crossed, and all this push and
energy is turning into channels of agitation.
- Frederick J. Turner, "The Problem of the West," 1896
O A. The completion of the Transcontinental Railroad
OB. The formation of the Immigration Restriction League
O c. The end of Reconstruction in the South
O D. The passage of the Indian Removal Act