Factory jobs were plentiful and available in the cities is the right answer.
Numerous factors were responsible for the growth of cities in the late 1800s in the United States, including the growth of factories, the need for jobs and the immigrants. Arriving from around the world these Immigrants helped for the population growth of cities, and the migration of rural American population (particularly farmers- as machines were used in many farms, moved to cities to find new jobs) led to a significant increase in the population of these cities. Chicago is one of the examples of a growing city during the late 1800s.
