In the late 1960's, many people in North America turned their attention to environmental problems, and new steel-and-glass skyscrapers were widely criticized. Ecologists pointing【B1】______that a cluster of tall buildings in a city often overburdens public transportation and parking lot【B2】______. Skyscrapers are also enormous【B3】______, and wasters, of electric power. In one recent year, the addition【B4】 17 million square feet of skyscraper office space in New York City raised the 【B5】______ daily demand for electricity by 120,000 kilowatts—enough to【B6】______the entire city of Albany for a day. Glass-wailed skyscraper can be especially【B7】______. The heat loss (or gain) through a wall of half-inch plate glass is more than ten times 【B8】______ through a typical masonry wall filled with insulation board. To lessen the strain【B9】______heating and air-conditioning equipment,【B10】______of skyscrapers have begun to use double-glazed panels of glass, and reflective glasses【B11】______with silver or gold mirror films that reduce【B12】______as well as heat gain. However,【B13】______skyscrapers raise the temperature of the surrounding air and【B14】______neighboring buildings. Skyscrapers put severe pressure on a city's sanitation【B15】______, too. If fully occupied, the two World Trade Center towers in New York City would alone generate 2.25 million gallons of raw sewage each year—as【B16】______as a city the size of Stamford, Connecticut, which has a【B17】______of more than 109,000. Skyscrapers also【B18】______with television reception, block bird flyways, and obstruct air traffic. Still, people【B19】______to build skyscrapers for all the reasons that they have always built them—personal ambition and the【B20】______of owners to have the largest possible amount of rentable space.