[单选题]
The following graph shows the concentration of
+gld dvp5 pr .+.qp+wvatmospheric ca
bpep(xh(t)g7gttu 3) rbon dioxide measured at Mauna Loa Observatory
g) bx (t 7e3t(h)gptupin Hawaii, USA.
Which of the following statements do not explain the rise and fall of the carbon dioxide levels shown in the graph above?
A. During the winter months, there is a decrease in photosynthesis due to lower temperatures and therefore, an overall increase in atmospheric carbon dioxide.
B. As temperatures rise in the summers, there is decreased fixation of carbon dioxide, causing a rise in atmospheric carbon dioxide.
C. The fluctuations are repeated every year, but the amplitude has been increasing due to the cumulative effects of increasing carbon dioxide from the combustion of fossil fuels.
D. Leaves and other detritus decay in winter months which releases carbon dioxide.
参考答案:
B
本题详细解析:
Explanation:
As temperatures rise in the summers, more carbon dioxide is fixed, causing a decrease in atmospheric carbon dioxide. There is more photosynthesis and more growth of vegetation. Overall, more carbon dioxide is removed from the atmosphere, causing a seasonal decrease. Therefore Choice B is not a correct explanation of the fluctuations of carbon dioxide, so is the correct answer.
During the winter months, photosynthesis decreases due to lower temperatures, leading to a decrease in the removal of carbon dioxide from the atmosphere which can contribute to increases in atmospheric carbon dioxide levels. Therefore Choice A is a correct explanation of fluctuating carbon dioxide levels, and so is not the correct answer choice. Fluctuations in carbon dioxide levels are indeed repeated every year due to natural seasonal cycles, but the amplitude of these fluctuations has been increasing due to the cumulative effects of human activities, particularly the combustion of fossil fuels, which releases additional carbon dioxide into the atmosphere. Choice C is therefore a true statement about fluctuating carbon dioxide levels and so is not the correct choice. The decay of leaves and detritus in the winter months can lead to the release of carbon dioxide as organic matter decomposes, contributing to
