In the late 1960's, many people in North America turned their attention to environmental p

roblems, and new steel-and-glass skyscrapers were widely criticized. Ecologists pointed out that a cluster of tall buildings in a city often overburdens public transportation and parking lot capacities.

Skyscrapers are also lavish 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 masonry wall filled with insulation board. To lessen the strain on heating and air-conditioning equipment, builders of skyscrapers have begun to use double-glazed panels of glass I 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(former) World Trade Center towers in New York City would alone generate 2.25 million gallons of raw sewage(污物) 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.

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

The main purpose of the passage is to ______.

A．discuss the advantages and disadvantages of skyscrapers

B．compare skyscrapers with other modern structures

C．describe skyscrapers and their effect on the environment

D．illustrate various architectural designs of skyscrapers