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. By opening vast areas of unoccupied land for residential expansion, the buses,【C2】______ , commuter trains, and electric trolleys pulled settled regions outward two to four times more【C3】______ from city centers than they were in the pre-modern era. In 1850, for example, the borders of Boston lay【C4】______ two miles from the old business district by the turn of the century the【C5】______ ex tended ten miles. Now those who could afford it could live far removed from the old city center and still【C6】______ there for work, shopping, and【C7】______ The new accessibility of land around the periphery (外围) of almost every, major city【C8】______ an explosion of real estate development and fueled what we now【C9】______ as urban sprawl (城市蔓延). Between 1890 and 1920, for example, some 250,000 new【C10】______ 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【C13】______ of the possibilities of commuting, real estate developers added 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】______ pre sent a feature of residential expansion【C16】______ to the growth of mass transportation: urban sprawl was【C17】______ unplanned. It was carried out by thousands of small investors who paid little care to coordinated land use or to future land users. Those who purchased and prepared land for residential purposes, particularly land near or outside city borders【C18】______ transit lines and middle-class inhabitants were anticipated, did so to create demand【C19】______ much as to respond to it. Chicago is a prime example of this【C20】______ . Real estate subdivision there proceeded much faster than population growth. 【C1】