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  3. Although many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they pose many potential hea

Although many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they pose many potential hea

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问题     Although many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they pose many potential health hazards. Much research has looked at how the movement of air inside a closed environment—such as an office building—can spread disease or expose people in the building to harmful chemicals.
    One of the more widely publicized dangers is that of Legionnaire’s disease, which was first recognized in the 1970s. This was found to have affected people in buildings with air-conditioning systems in which warm air pumped out of the system’s cooling towers was somehow sucked back into the air intake(通风口), in most cases due to poor design. The warm air, filled with bacteria, was combined with cooled, conditioned air and was then circulated around various parts of the building. Studies showed that even people outside such buildings were at risk if they walked past air exhaust pipes.
    Large air-conditioning systems add water to the air they circulate by means of humidifiers(湿度调节器). In older systems, the water used for this process is kept in special reservoirs, the bottoms of which provide breeding grounds for bacteria which can find their way into the ventilation(通风)system. The risk to human health from this situation has been highlighted by the fact that the immune systems of approximately half of workers in air-conditioned office buildings have developed the ability to fight off the organisms found at the bottom of system reservoirs. But chemicals called "biocides" are added to reservoirs to make them germ-free, and they are dangerous in their own right in sufficient quantities, as they often contain compounds strongly linked to cancers.
    Finally, it should be pointed out that the artificial climatic environment created by air-conditioners can also affect us. In a natural environment, whether indoor or outdoor, there are small variations in temperature and humidity. Indeed, the human body has long been accustomed to these normal changes. In an air-conditioned living or work environment, however, body temperatures remain well under 37℃, our normal temperature. This leads to a weakened immune system and thus greater exposure to diseases such as colds and flu.
The last paragraph implies that our immune system can be weakened when _____.
选项 A、we live in an artificial climatic environment
B、there are variations in temperature and humidity
C、our body temperatures often remain not high enough
D、we are often exposed to diseases such as colds and flu
答案C
解析 本题考查代词的理解。解题的关键在于理解最后一句中的This是指上一句所说的情况。A中所说的人造环境不是使免疫系统变弱的必然条件。B是保持免疫系统健康的条件。D是免疫系统变弱后的影响。
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