Climatic conditions are delicately adjusted to composition of the Earth's atmosphere. If there were a change in the atmosphere -- for example, in the   amounts of atmospheric gases -- the climate would probably change also. A slight increase in water vapor   , for example, would increase the heat-keeping capacity of the atmosphere and would lead to a rise in global temperatures. The level of carbon dioxide, CO2, in the atmosphere has an important effect on climatic change. Most of the Earth's incoming energy is short-wavelength radiation   , which tends to pass through atmospheric CO2easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation  , which CO2absorbs and then goes toward the Earth. 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 CO2, whose surface temperatures have been measured at 430~C.If the CO2 content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO2concentration were halved, the Earth would become completely covered with ice.  Another equally respectable theory, however, states that a halving of the CO2concentration would lead only to a reduction in global temperatures of 3degC. If, because of an increase in forest fires or volcanic activity, the CO2content of the atmosphere increased, a warmer climate would be produced. Plant growth, which relied on both the warmth and the availability of CO2, would probably increase. As a consequence, plant would use more and more CO2.  Eventually CO2levels would decrease and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO2would thus 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 CO2present in the atmosphere, with regular temperature increase and decrease. Some climatologists argue that the burning of fossil fuels has raised the level of CO2in the atmosphere and has caused a global temperature increase of at least 1degC. But a supposed global temperature rise of 1 degC may in reality be only several regional temperature increases, restricted to areas where there are many meteorological   stations. Other areas, for example, the Southern Hemisphere oceanic zone, may be experiencing an equal temperature decrease that is unrecognized because of the shortage of meteorological recording stations. The author is primarily concerned with   _  .
explaining the effects that the burning of fossil fuels might have on climate explaining the effects of CO2on atmospheric radiation discussing effects that changes in the CO2level in the atmosphere might have on climate challenging hypotheses    about the effects of water vapor and CO2on climate
Question: Climatic conditions are delicately adjusted to composition of the Earth's atmosphere. If there were a change in the atmosphere -- for example, in the   amounts of atmospheric gases -- the climate would probably change also. A slight increase in water vapor   , for example, would increase the heat-keeping capacity of the atmosphere and would lead to a rise in global temperatures. The level of carbon dioxide, CO2, in the atmosphere has an important effect on climatic change. Most of the Earth's incoming energy is short-wavelength radiation   , which tends to pass through atmospheric CO2easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation  , which CO2absorbs and then goes toward the Earth. 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 CO2, whose surface temperatures have been measured at 430~C.If the CO2 content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO2concentration were halved, the Earth would become completely covered with ice.  Another equally respectable theory, however, states that a halving of the CO2concentration would lead only to a reduction in global temperatures of 3degC. If, because of an increase in forest fires or volcanic activity, the CO2content of the atmosphere increased, a warmer climate would be produced. Plant growth, which relied on both the warmth and the availability of CO2, would probably increase. As a consequence, plant would use more and more CO2.  Eventually CO2levels would decrease and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO2would thus 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 CO2present in the atmosphere, with regular temperature increase and decrease. Some climatologists argue that the burning of fossil fuels has raised the level of CO2in the atmosphere and has caused a global temperature increase of at least 1degC. But a supposed global temperature rise of 1 degC may in reality be only several regional temperature increases, restricted to areas where there are many meteorological   stations. Other areas, for example, the Southern Hemisphere oceanic zone, may be experiencing an equal temperature decrease that is unrecognized because of the shortage of meteorological recording stations. The author is primarily concerned with   _  .
A. explaining the effects that the burning of fossil fuels might have on climate
B. explaining the effects of CO2on atmospheric radiation
C. discussing effects that changes in the CO2level in the atmosphere might have on climate
D. challenging hypotheses    about the effects of water vapor and CO2on climate
Answer: