How did the Central Pacific and Union Pacific railroad companies profit from constructing the railroad? by selling railroad lines back to the government at a profit
by owning businesses in the towns that developed along the railroad
by building railroad monopolies and continually raising prices
by selling pieces of land granted to them to build the railroad

By selling land given to them to build the railroad

Explanation:

The government gave the railroad companies large amounts of land to sell both so that the railroad companies earn some money and so they can speed up the settling of the frontier.

D.) By selling pieces of land granted to them to build the railroad.

Explanation:

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the constitution of the united states gave congress the absolute authority over immigration laws