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　　Ⅰ.阅读理解

　　(2016·河北普通毕业班质量检查)The term “smog” was first used in London during the early 1900's to describe the combination of smoke and fog. What we typically call “smog” today is a mixture of pollutants but is primarily made up of ground­level ozone (臭氧)．

　　Ozone can be beneficial or harmful depending on where it stays. The ozone staying high above the Earth protects human health and the environment,

　　but ground­level ozone is responsible for the choking, coughing, and painful eyes associated with smog. The ozone in smog also prevents plants growth and can cause widespread damage to crops and forest.

　　Major smog occurrences often are linked to heavy motor vehicle traffic, high temperatures, sunshine, and calm winds. Weather and geography affect the position and severity of smog. Because temperature regulates the length of time it takes for smog to form, smog can form faster and be more severe on a hot and sunny day. When warm air stays near the ground instead of rising and winds are calm, smog may stay trapped over your city for days. As traffic and other sources add more pollutants to the air, the smog gets worse. Smog is often more severe away from the pollution sources because the chemical reactions that cause smog occur in the atmosphere while the reacting chemicals are being moved by the wind.

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