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【C1】______that a cluster of tall buildings in a city often overburdens public transportation and parking lot【C2】______. Skyscrapers are also enormous【C3】______, and wasters, of electric power. In one recent year, the addition 【C4】______17 million square feet of skyscraper office space in New York City raised the【C5】______daily demand for electricity by 120,000 kilowatts—enough to【C6】______the entire city of Albany for a day. Glass-wailed skyscraper can be especially【C7】______. The heat loss (or gain) through a wall of half-inch plate glass is more than ten times【C8】______through a typical masonry wall filled with insulation board. To lessen the strain【C9】______heating and air-conditioning equipment, 【C10】______of skyscrapers have begun to use double-glazed panels of glass, and reflective glasses【C11】______with silver or gold mirror films that reduce【C12】______as well as heat gain. However, 【C13】______skyscrapers raise the temperature of the surrounding air and【C14】______neighboring buildings. Skyscrapers put severe pressure on a city"s sanitation【C15】______, 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【C16】______as a city the size of Stamford, Connecticut, which has a【C17】______of more than 109,000. Skyscrapers also【C18】______with television reception, block bird flyways, and obstruct air traffic. Still, people【C19】______to build skyscrapers for all the reasons that they have always built them—personal ambition and the【C20】______of owners to have the largest possible amount of rentable space.