After the completion of the railroad, a major industry in the West was ranching. Thus the correct answer is A.
What is Industry?
The industry refers to an area where the activity of manufacturing or production of goods and their assembly will be done in order to distribute to intermediaries to serve the customers.
Railroad helps to travel from one location to another in a quick manner and helps in exporting raw materials as well as goods. It is the cheapest and safest way to move in ancient times.
Ranching is the practice of maintaining animal grazing on large tracts of land. Ranchers commonly rear grazing animals like sheep and cattle. Cattle can be transported from one location to another using railroad.
The geography of the West made the entire region suitable to be turned into a grazing area and the Great Plains made it possible for people to start farming.
Therefore, option A ranching is appropriate.
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