In the atmosphere, carbon dioxide acts rather like a one-mirror--the glass in the roof of

a green-house which allows the sun' s rays to enter but prevents the heat from escaping.

According to a weather expert' s prediction, the atmosphere will be warmer in the year 2050 than it is today, if man continues to burn fuels at the present rate. If this warming up took place, the ice caps in the poles would begin to melt, thus raising sea level several meters and severely flooding coastal cities. Also, the increase in atmospheric temperature would lead to great changes in the climate of the northern hemisphere, possibly resulting in an alteration of the earth' s chief food-growing zones.

In the past, concern about a man-made warming of the earth has concentrated on the Arctic because the Antarctic is much colder and has a much thicker ice sheet. But the weather experts are now paying more attention to West Antarctic, which may be affected by only a few degrees of warming: in other words, by a warming on the scale that will possibly take place in the next fifty years from the burning of fuels.

Satellite pictures show that large areas of Antarctic ice are already disappearing. The evidence available suggests that a warming has taken place. This fits the theory that carbon dioxide warm the earth.

However, most of the fuel is burnt in the northern hemisphere, where temperatures seem to falling. Scientists conclude, therefore, that up to now natural influences on the weather have exceeded those caused by man. The question is: Which natural cause has most effect on the weather？

One possibility is the variable behavior. of the sun. Astronomers at one research station have studied the hot spots and "cold" spots (that is, the relatively less hot spots) on the sun. As the sun rotated, every 27.5 days, it presents hotter of "colder" faces to the earth, and different aspects to different parts of the earth. This seems to have a considerable effect on the distribution of the earth' s atmospheric pressure, and consequently on wind circulation. The sun is also variable over a long term: its heat output goes up and down in cycles, the latest trend being downward.

Scientists are now finding mutual relations between models of solar-weather interactions and the actual climate over many thousands of years, including the last Ice Age. The problem is that the models are predicting that the world should be entering a new Ice Age and it is not. One way of solving this theoretical difficulty is to assume a delay of thousands of years while the solar effects overcome the inertia of the earth' s climate. If this is right, the warming effect of carbon dioxide might thus be serving as a useful counter-balance to the sun's diminishing heat.

It can be concluded that a concentration of carbon dioxide in the atmosphere would ______ .

A．prevent the sun' s rays from reaching the earth' s surface

B．mean a warming up in the Arctic

C．account for great changes in the climate in the northern hemisphere

D．raise the temperature of the earth' s surface