单选题 请阅读 Passage 1，完成第 21~25小题。Passage 1These 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 advantage of Enwave 's new system?

单选题 Passage2People often complained about not getting"a good night's sleep"，but sleep patterns differ from person to person. Most adults require six to eight hours of sleep to function well，while others survive on only a few hours. Still，most people today think of sleep as one continuous period of downtime. This is not the way people used to sleep. According to researchers in earlier times，people divided sleep by first sleep a few hours，waking up，then going back to sleep.Before the 18th century，people had no gas or electricity in their homes. Fire，candles，or oil lamps were the common forms of lighting. This lack of artificial lighting in homes contributed to people's sleep patterns. It made sense for people to go to bed early. If you live in this time period，you might be a hard-working farmer，and you would come home，eat and quickly fall into bed exhausted. You would probably go to sleep at 9：00 or 10：00 P.M. In this first period of sleep-called first sleep-you would typically sleep until midnight or shortly afterwards.Halfway through the night during a period some call the watch，or watching period. When you came out of first sleep，you would stay in bed and relax quietly. You might talk with a bedfellow，meditate on the day's events or the meaning of a dream，or just let your mind wander. If you enjoyed writing or drawing，you might get out of bed to write a poem or story or draw a picture.Then you would start to feel sleepy, so would return to bed and fall asleep again for your second sleep. This period would continue until early morning when daylight arrived. Again, with no artificial lighting in homes, people naturally woke up early to take advantage of sunlight.Today, human may consider divided sleep a strange habit, but sleep researchers say that it is actually a more natural sleep pattern. Dr. Thomas Wehr of the U.S. National Institute of Mental Health has studied human sleep. He thinks that modern sleep problems occur because the orderly, natural way of sleep is breaking through the more recent continuous sleep pattern. Wehr and other scientists believe that artificial lighting has altered the way people sleep. In a research study, he asked 15 adults to rest and sleep in darkness for 14 hours(from 6:00 P.M. to 8:00 A.M.). At first, the subjects took a few hours to get to sleep, and then slept 11 hours a night. Then overtime, they switched to divided sleep. They fell asleep for about 3 or 5 hours in the evening, stay awake for an hour of two and then slept again for four hours till early morning.Unlike the people in the study, we modern humans generally do not practice divided sleep. However, many of us have the experience of waking up in the middle of the night. We usually consider this a sleeping "problem", but perhaps we should look at it as natural behavior. Divided sleep may be the way we should all be sleeping. A first sleep followed by a relation period and a second period of sleep could help all of us to beat the stress of our fast-paced lives.What is the main idea of the text?

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