Read the following passage and choose the best answer (A, B, C, D):
The principal difference between urban growth in Europe and in the North American colonies was the slow evolution of cities in the former and their rapid growth in the latter. In Europe they grew over a period of centuries from town economies to their present urban structure. In North America, they started as wilderness communities and developed to mature urbanism in little more than a century. In the early colonial days in North America, small cities sprang up along the Atlantic Coastline, mostly in what are now New England and the Middle Atlantic states in the United States and in the lower Saint Lawrence valley in Canada. This was natural because these areas were nearest England and France, particularly England, from which most capital goods were imported. Merchandising establishments were, accordingly, advantageously located in port cities from which goods could be readily distributed to interior settlements. Here, too, were the favored locations for processing raw materials prior to export. Boston, Philadelphia, New York, Montreal, and other cities flurished, and as the colonies grew, these cities increased in importance. This was less true in the colonial South, where life centered around large farms, known as plantations, rather than around towns, as was the case in the areas further north along the Atlantic coastline. The local isolation and the economic self-sufticiency of the plantations were antagonistic to the development of the towns. The plantations maintained their independence because they were located on navigable streams and each had a wharf accessible to the small shipping of that day. In fact, one of the strongest factors in the selection of plantation land was the desire to have it front on a water highway. When the United States became an independent nation in 1776, it did not have a single city as large as 50,000 inhabitants, but by 1820 it had a city of more than 100,000 people, and by 1880 it had recorded a city of over one million. It was not until after 1823, after the mechanization of the spinning and weaving industries, that cities started drawing young people away from farms. Such migration was particularly rapid following the Civil War (1861 -1865)
2. The word "they" in paragraph 1 refers to .
