题目内容：
根据材料，回答问题。Effects of Environmental Pollution
If pollution continues to increase at the present rate, formation of aerosols (浮质) in theatmosphere will cause the onset (开始) of an ice age in about fifty years' time.Thisconclusion, reached by Dr S. I. Rasool and Dr S. H. Schneider of the United States GoddardSpace Flight Centre, answers the apparently conflicting questions of whether an increase in thecarbon dioxide (二氧化碳) content of the atmosphere will cause the Earth to warm up orincreasing the aerosol content will cause it to cool down.The Americans have shownconclusively that the aerosol question is dominant.
Two spectres haunting conservationists have been the prospect that environmental pollutionmight lead to the planet's becoming unbearably hot or cold. One of these ghosts has now beenlaid. because it seems that even an increase in the amount of carbon dioxide in the atmosphereto eight times its present value will produce an increase in temperature of only 2℃.whichwould take place over several thousand years. But the other problem now looms larger thanever.
Aerosols are collections of small liquid or solid particles dispersed in air or some othermedium. The particles are all so tiny that each is composed of only a few hundred atoms.Because of this they can float in the air for a very long time. Perhaps the most commonlyexperienced aerosol is industrial smog (烟雾) of the kind that plagued London in the 1950sand is an even greater problem in Los Angeles today. These collections of aerosols reflect theSun's heat and thereby cause the Earth to cool.
Dr. Rasool and Dr. Schneider have calculated the exact effect of a dust aerosol layer justabove the Earth's surface in the temperature of the planet. As the layer builds up, the presentdelicate balance between the amount of heat absorbed from the Sun and the amount radiated fromthe Earth is disturbed. The aerosol layer not only reflects much of the Sun's light but alsotransmits the infrhred (红外线的) radiation from below. So, while the heat input to thesurface drops, the loss of heat remains high until the planet cools to a new balanced state.
Within fifty years, if no steps are taken to stop the spread of aerosols in the atmosphere, acooling of the Earth by as much as 3.5℃. seems inevitable. If that lasts for only a few years itwould start another ice age, and because the growing ice caps at each pole would themselvesreflect much of the Sun's radiation it would probably continue to develop even if the aerosollayer were destroyed.
The only bright spot in this gloomy forecast lies in the hope expressed by Dr. Rasool and Dr. Schneider that nuclear power may replace fossil fuels in time to prevent the aerosol contentof the atmosphere from becoming critical.
The author's main purpose in writing the article is to warn of__________ A．warm weather
B．hot weather
C．a new ice age
D．a new iceberg
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