Many people once thought that air pollution had a bad influence upon only the area immediately around large cities with factories and heavy automobile traffic. Today, we know that although these are the areas with the worst air pollution, the problem is nearly worldwide. On several occasions over the past decade, a heavy cloud of air pollution has covered the entire eastern half of the United States and led to health warnings even in rural areas away from any major concentration of factories and automobile traffic. In fact, the very climate of the entire earth may be affected by air pollution. Some scientists feel that the increasing concentration of carbon dioxide( 二氧化碳) in the air resulting from the burning of fossil fuels (coal and oil) is creating a“greenhouse effect” － holding in heat reflected from the earth and raising the world’s average temperature. If this view is correct and the world’s temperature is raised only a few degrees, much of the polar ice cap will melt and cities such as New York, Boston, Miami, and New Orleans will be under water.
Another opinion, less widely held, is that increasing tiny substances in the atmosphere are blocking sunlight and lowering the earth’s temperature－a result that would be equally disastrous.A drop of just a few degrees could create something close to new ice age and would make agriculture difficult or impossible in many of our top farming areas. At present we do not know for sure that either of these conditions will happen (though one recent government report prepared by experts in the field concluded that the greenhouse effect is very likely). Perhaps, if we are very lucky, the two tendencies will make up for each other and the world’s temperature will stay about the same as it is now.
As pointed out at the beginning of the passage, people used to think that air pollution _____.

A．caused widespread damage in the countryside

B．affected the entire eastern half of the United States

C．almost brought worldwide effect

D．existed merely in urban and industrial areas

As far as the greenhouse effect is concerned, the author _____________.

A．shares the same view with the scientists

B．is uncertain of its consequence

C．rejects it as being ungrounded

D．thinks that it will increasingly destroy the world

It can be inferred from the passage that ____________.

A．raising the world’s temperature a little would not do much harm to life on earth

B．lowering the world’s temperature a little would lead to agricultural disasters

C．almost no temperature changes have occurred over the past decade

D．the world’s temperature will remain constant in the years to come

This passage is primarily concerned with _____________.

A．the greenhouse effect in the world

B．the measures to adjust the climate

C．the potential effect of air pollution

D．the measures to protect the environment