Answer:
The opportunity to work in the cattle industry; to be a “cowboy” Faster travel to the West by railroad; availability of supplies due to the railroad. The opportunity to own land cheaply under the Homestead Act. The discovery of wheat strains adapted to grow in the climate of the Plains.
Four empires collapsed: the Russian Empire in 1917, the German and the Austro-Hungarian in 1918
I would have to say that A is your best choice.