Passage 1

These days,many large city buildings are equipped with their own air-conditioning systems.These systems help keep the buildings cool,but they can also damage the environment.Since they?use a lot of electricity,for instance,they contribute indirectly to global warming.In addition,the?water that flows through the systems is often cooled using chemicals called chlorofluorocarbons,or?CFCs,that are believed to damage the Earth′s ozone layer.Recently,though,a system has been?built in the city of Toronto,Canada,that cools buildings with little damage to the environment.

In the traditional air-conditioning systems found in most large buildings,water is pumped?through the building in a continuous cycle.The water is first cooled to a temperature of 4℃in?machines called chillers.It is then sent to individual units that cool the air in each room.As the?water flows through the building,it gradually becomes warmer.Finally,it reaches the roof,where it?is left to cool down naturally in a water tower.After that it is returned to the chillers,where the cycle?begins again.

Toronto lies on the shore of Lake Ontario,one of North America′s Great Lakes,and the new?system makes use of cold water taken from about 80 meters below the surface of the lake.At this?depth,the water in the lake remains at 4℃all year round.This is exactly the temperature to which?the water in air-conditioning systems is cooled.However,the water from the lake is not pumped?directly into the air-conditioning systems.Instead,it is used to cool the water that is already inside?the air-conditioning systems.After that,the lake water is added to the city′s ordinary water supply.

Enwave,the company that developed this deep-lake cooling system,says that it uses 75 percent?less energy than traditional air conditioning.And since no CFCs are used,no damage can be caused?to the ozone layer.Not every city is located next to a large lake,but experts believe that systems like?the one being used in Toronto could be built elsewhere by using other natural sources of cold water.

What is one problem with traditional air-conditioning systems?查看材料

• A.The chemicals they use sometimes pollute the water supply
• B.They often damage the buildings that they are trying to cool
• C.There are not enough CFCs in the world to cool all the buildings
• D.The method they use to cool water is thought to harm the ozone layer