Passage Three

Questions 31 to 35 are based on the following passage.

In the late 1960's many people in North America turned their attention to environmental problems，and new steel-and-glass skyscrapers(摩天楼)were widely criticized. Scientists pointed out that blocks of tall buildings in a city often overburdens. public transportation and parking lot capacities.

Skyscrapers are also big consumers，and wasters，of electric power. In one recent year. the addition of 17 million square feet of skyscraper office space in New York City raised the peak daily demand for electricity by 120，000 kilowatts-enough to supply the entire city of Albany，New York，for a day.

Glass-walled skyscrapers can be especially wasteful. The heat loss(or gain)through a wall of half-inch plate glass is more than ten times that through a typical brick wall filled with insulation board. To lessen the strain on heating and air-conditioning equipment，builders of skyscrapers have begun to use double-glazed(装上玻璃的)panels of glass，and reflective glasses coated with silver or gold mirror films that reduce glare as well as heat gain. However. mirror-walled skyscrapers raise the temperature of the surrounding air and affect neighboring buildings.

Skyscrapers put a severe strain on a city's sanitation(卫生)facilities，too. If fully occupied, the two World Trade Center towers in New York City would alone generate 2.25 million gallons of raw wastes each year-as much as a city the size of Stamford，Connecticut，which has a population of more than 109，000.

Skyscrapers also interfere with television reception，block bird flyways，and obstruct air traffic. In Boston in the late 1960's，some people even feared that shadows from skyscrapers would kill the grass on Boston Common.

Still，people continue to build skyscrapers for all the reasons that they have always built them-personal ambition(抱负)pride，and the desire of owners to have the largest possible amount of rentable space.

 The main purpose of the passage is to______.