Answer:
C. Construction of railroads
Explanation:
Both the United States and Canada had early settlements in the eastern parts of these two countries, but it was until railroads began to be built in the early ninenteenth century, that the central and eastern parts of these countries began to be settled.
In the United States for example, railroads were crucial to spread settlers in the Midwest, the Plains states, the Mountain West, and the West Coast. Without railroads, settlements of these areas would have been much slower and much more sparse.
