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  3. Smog is something combined by smoke and fog. London is always known for its "black fogs". In the winter of 1952, a milky white f

Smog is something combined by smoke and fog. London is always known for its "black fogs". In the winter of 1952, a milky white f

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问题    Smog is something combined by smoke and fog. London is always known for its "black fogs". In the winter of 1952, a milky white fog rolled into the city. It soon turned into black smog as the smoke of the city poured into the air. When it was over, more than 4,000 people had been killed by the thick black smog.
   New York City has had several London type smog since 1950. Each time there were from 100 to 400 deaths caused by the smog.
   In all the killer smog, factories and homes poured smoke and fumes into the air from the furnaces. The chemical fumes combined with the water-droplets in the fog to form harmful substances. These substances caused the illness of those who breathed the polluted air.
   Usually, such harmful fumes rise into the upper air and are blown away by the wind. But sometimes, there is an unusual weather condition called a temperature inversion. A layer of cold air remains near the ground as smoke and fumes pour into it. This is covered by an upper layer of warm air that acts like a lid. It prevents the polluted cooler air rising. The harmful fumes piles up and make people iii. These fumes contain sulfur dioxide, soot particles, and other chemicals. The smog may be so thick that airports are closed and chains of collisions occur on the highways.
   Another type of smog occurs in Los Angeles. Here the weather may be clear and sunny, but stinging eyes and dry coughs show that harmful chemicals fill the air. The smog is due to invisible gases, mostly from automobile exhausts. Because these chemicals are changed by the sun up in the air, Los Angeles smog is called Photochemical smog. It contains automobile exhaust fumes and nitrogen oxides changed by the sun’ s rays. Added to these are sulfur dioxide and other fumes from factories of oil refineries. Photochemical smog is found in many large cities all over the world.
   Killer smog doesn’ t happen very often, fortunately. But in many large cities, a combination of automobiles exhaust fumes, home furnace smoke, and factory waste gases pours into the air. This may also happen in the suburbs in the country, where large factories have been built. A number of harmful substances have affected the health of the population.  
Killer smog will also happen in the suburbs if ______ .
选项 A、there are too many cars
B、it is too densely populated
C、it is always cloudy
D、too many factories are built
答案D
解析 见文章第五段结尾。
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