Mass transportation revised the social and economic fabric of the American city in three fundamental ways. It speeded up physical expansion, it sorted out people and land uses, and it【C1】______the inherent instability of urban life.【C2】______opening vast areas of unoccupied land for residential expansion, the buses,【C3】______. commuter trains, and electric trolleys pulled settled regions outward two to four times more【C4】______from city centers than they were in the pre-modern era In 1850, for example, the borders of Boston lay【C5】______two miles from the old business district; by the turn of the century the【C6】______extended ten miles. Now those who could afford it could live far removed from the old city center and still【C7】______there for work, shopping, and【C8】______. The new accessibility of land around the periphery of almost every major city【C9】______an explosion of real estate development and fueled what we now【C10】______as urban sprawl. Between 1890 and 1920, for example, some 250,000 new residential lots were recorded within the borders of Chicago,【C11】______of them located in outlying areas. Over the same period, another 550,000 were plotted outside the city limits【C12】______within the metropolitan area. Anxious to take advantage of the possibilities of commuting, real estate developers【C13】______800,000 potential building sites to the Chicago region in just thirty years—lots that could have housed five to six million people. Of course, many were never【C14】______; there was always a huge surplus of subdivided, but vacant, land around Chicago and other cities. These【C15】______present a feature of residential expansion【C16】______the growth of mass transportation: urban sprawl was essentially unplanned. It was【C17】______by thousands of small investors who paid little care to coordinated land use or to【C18】______land users. Those who purchased and prepared land for residential purposes, particularly land near or outside city borders【C19】______transit lines and middle-class inhabitants were anticipated, did so to create demand as much as to respond to it. Chicago is a prime example of this【C20】______. Real estate subdivision there proceeded much faster than population growth.