【单选题】

Climatic conditions are delicately adjusted to the composition of theEarth’s atmosphere. If there were a change in the atmosphere--for example, in the relative proportions of atmospheric gases"-the climate would probably change also.

A、slight increase in water vapor, for instance, would increase the heat-retaining capacity of the atmosphere and would lead to a rise in global temperatures. In contrast, a large increase in water vapor would increase the thickness and extent of the cloud layer, reducing the amount of solar energy reaching theEarth’s surface.
The level of carbon dioxide in the atmosphere has an important effect on climatic change. Most of theEarth’s incoming energy is short-wavelength radiation, which tends to pass through atmospheric carbon dioxide easily. TheEarth, however, reradiates much of the received energy as long-wavelength radiation, which carbon dioxide absorbs and then remits toward theEarth. This phenomenon, known as the greenhouse effect, can result in an increase in the surface temperature of a planet.An extreme example of the effect is shown by Venus, a planet covered by heavy clouds composed mostly of carbon dioxide, whose surface temperatures have been measured at 43012. If the carbon dioxide content of the atmosphere is reduced, the temperature falls.According to one respectable theory, if the atmospheric carbon dioxide concentration were halved, theEarth would become completely covered with ice.Another equally respectable theory, however, states that a halving of the carbon dioxide concentration would lead only to reduction in global temperatures of 312.
If, because of an increase in forest fires or volcanic activity, the carbon dioxide content of the atmosphere increased, a warmer climate would be produceD、Plant growth, which relies on both the warmth and the availability of carbon dioxide, would probably increase.As a consequence, plants would use more and more carbon dioxide.Eventually carbon dioxide levels would diminish and the climate, in turn, would become cooler. With reduced temperatures many plants would die; carbon dioxide would thereby be returned to the atmosphere and gradually the temperature would rise again. Thus, if this process occurred, there might be a long-term oscillation in the amount of carbon dioxide present in the atmosphere, with regular temperature increases and decreases of a set magnitude.
Some climatologists argue that the burning of fossil fuels has raised the level of carbon dioxide in the atmosphere and has caused a global temperature increase of at least 1C、But a supposed global temperature rise of 112 may in reality be only several regional temperature increases, restricted to areas where there are many meteorological stations and mused simply by shifts in the pattern of atmospheric circulation. Other areas, for example, the Southern Hemisphere Oceanic Zone, may be experiencing an equivalent temperature decrease that is unrecognized because of the shortage of meteorological recording stations.
All of the following can be found in the author’s discussion of climate except______.
A、an indication of the effect of an increase in water vapor in the atmosphere
B．a contrast between two theories about the effects of a lowering of carbon dioxide levels in the atmosphere
C．a generalization about the efficiency of meteorological recording stations
D．hypothesis about the relationship between atmospheric gases and changes in climate