Which of the following was a reason for the growth of cities in the late 1800s?

When the different ports of Boston, New York, Philadelphia, and Baltimore opened in the 1800s cities started to grow. It transformed the demographic landscape of America. Manufacturing was the main source of the income America’s income. In the late 1700s, there were only 12 cities in the US but by the time the ports were opened 16% of Americans lived in cities. 

really and truely i dont know sorry