Hm, I'd have to say "Without innovations in food production, the factories could not have grown so quickly. The trend quickened after 1860 and continued throughout the 21st century as well."
Explanation:
In 1820, the United States had only a few cities of 10,000 residents or more. About seven percent of U.S. residents lived in urban areas. The number of cities with more than 10,000 people grew quickly over the next 40 years, especially in the Northeast and Midwest. By 1860, about 20 percent lived in cities. Philadelphia and New York City were the most populated cities in 1860 and would soon reach one million residents.
