Which of the following, if true, would most seriously weaken the conclusion that a greenhouse effect will not result from the current release of large amounts of carbon dioxide into the atmosphere? A. The expected rise in average global temperatures has not yet been observed. B. Ocean waters absorb carbon dioxide at a greater rate when the atmospheric concentration of carbon dioxide is higher. C. Since the beginning of the Industrial Revolution, increased atmospheric concentrations of carbon dioxide have resulted in improved agricultural productivity. D. When plants decay, they produce methane, another gas that can have a marked greenhouse effect. E. The fact that carbon dioxide levels have risen and fallen many times in the Earth's history suggests that there is some biological process that can reverse the greenhouse effect.