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Below are the choices and the one with x mark is the answer:

allowing slaves to vote.
[X] building a transcontinental railroad.
letting popular sovereignty take its course.
ceding land back to Mexico.

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Correct answer:

Building a transcontinental railroad

An engineer named Theodore Judah had the idea of building a railroad through the Sierra Nevada mountains. Judah had recruited a group of investors in Sacramento to form the Central Pacific Railroad Company. This idea was presented in Congress and President Abraham Lincoln who promulgated the Pacific Railroad Law.

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