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.

Which details are used to support the main fact in the paragraph?

A) Mentions of Great Britain
B) Reasons why people move
C) Explanation of urbanization
D)Data about population changes