The correct answer is A. Grew as more and more people left rural areas
Explanation:
During the 19th century (the 1800s) the U.S. economy and society went through important changes, this can be explained as the First Industrial Revolution began in 1760 and led to new manufacturing processes based on machines and technology that replaced all hand production methods and promoted economies based on industry rather than on agriculture. Because of this, factories were built on cities or urban areas or near to them, this made people who live in rural areas move to the cities or urban areas as in these they could find job opportunities, indeed before the 19th century most of the population in the U.S. lived in rural areas but after the expansion of industries most of the population lived in rural areas, this made cities grow rapidly as thousands of people from rural areas and even immigrants from other countries arrived at them. Considering this, it can be concluded throughout the 1800s urban areas in the United States "grew as more and more people left rural areas".
