【单选题】

The passage supplies information for answering which of the following questions
A．Why are projections of the effects of changes in water vapor levels on the climate so inaccurate

B．What are the steps in the process that takes place as CO₂ absorbs long-wavelength radiation
C．How might our understanding of the greenhouse effect be improved if the burning of fossil fuels were decreased
D．What might cause a series of regular increases and decreases in the amount of CO₂ in the atmosphere