Final answer:
During the late 1800s and early 1900s, the population of the United States primarily changed as people moved from rural areas to urban areas.
Step-by-step explanation:
The population of the United States primarily changed during the late 1800s and early 1900s as people moved from rural areas to urban areas. This period, known as the Industrial Revolution, saw a rapid growth of cities and industries. Many people left their farms and agricultural jobs to seek better opportunities and higher wages in the cities.
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