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  3. Most people are aware that outdoor air pollution can damage their health, but many do not know that indoor air pollution can als

Most people are aware that outdoor air pollution can damage their health, but many do not know that indoor air pollution can als

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问题     Most people are aware that outdoor air pollution can damage their health, but many do not know that indoor air pollution can also【67】health significantly. Environmental Protection Agency studies【68】that indoor levels of pollutants may be 2-5 times, and【69】more than 100 times, higher than outdoor levels.
    There are many sources of indoor air pollution in any【70】. These include combustion (燃烧) sources, products for home cleaning and【71】, personal care, or hobbies;  central heating and cooling systems and humidification【72】; and outdoor air pollution.
  【73】effects may show up after a single exposure or【74】exposures. These include irritation of the eyes, nose, and throat, headaches, dizziness, and【75】. Such effects are usually short-term and【76】. Sometimes the treatment is simply【77】the person’s exposure to the source of the pollution,【78】it can be identified.【79】of some diseases may also show up soon after exposure to some indoor air pollutants.
    Certain effects are similar to those from colds or other viral (病毒引起的) diseases,【80】it is often difficult to【81】if the symptoms are a result of exposure to indoor air pollution.【82】this reason, it is important to pay attention to the time and place the symptoms【83】. If the symptoms fade or go away when a person is away from home and return when the person returns,【84】should be made to identify indoor air sources that may be possible【85】. Some effects may be made worse by an【86】supply of outdoor air or from the heating, cooling, or humidity conditions prevalent in the home.  
选项 A、wherever
B、whenever
C、whether
D、if
答案D
解析 此从句中的it是指the source of the pollution，选if表示“如果能够识别出污染源”。
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