答案　D　[推理判断题。本文介绍了一个教学实验：成绩相同的两个班由于分别接受积极的和消极的外部影响一年以后两个班的成绩出现明显差距。因此这项实验证明了外部影响的重要性。]

　　B

　　d to see that people had to walk in front of the buses to guide them.In this way, the most serious air pollution disaster in history began.When it was over, more than 4，000 people had been killed by the thick black smog.

　　New York City has had several London­type smogs since 1950.Each time，100 to 400 deaths were caused by the smog.Although these smogs were not as deadly as London's, New York City has the worst air pollution problem in the United States.

　　In all the killer smogs, 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 from rising.The harmful fumes pile up and make people ill.The smog may be so thick that airports are closed and chains of collisions occur on the highways.

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