Answer:
Louisiana Purchase, Texas Annexation, the Mexican Cession, the Erie Canal, the Homestead Act, the transcontinental railroad and the California Gold Rush are some examples of how the US started expanding its territory in the 1800s.
Explanation:
The most accurate words to fill the spaces would be:
Sugar plantations were the backbone ( source ) of the economy.
In 1890, the US Congress passed the ( McKinley Tariff ), which removed tariffs on all foreign sugar imports.
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The Mayan civilization was surrounded by mountains, and Harappa was not.
The Mayans worshipped animals, while the Harappans only worshipped gods.
Both were constantly at war.
