A large amount of carbon dioxide released into the atmosphere by burning fossil fuels now does not actually cause a greenhouse effect--that is, an increase in global average temperature. Because if the supply of carbon dioxide rises, plants will consume the gas more, so they will grow taller and lusher, so the concentration of carbon dioxide in the atmosphere will eventually remain stable. Which of the following items, if correct, will most seriously weaken the conclusion that a large amount of carbon dioxide currently released into the atmosphere will not cause the greenhouse effect. ['When the concentration of carbon dioxide in the atmosphere rises, seawater absorbs more carbon dioxide.', 'Methane is produced when plants rot, which is another gas that can cause a significant greenhouse effect.', 'From the beginning of the Industrial Revolution, the rise in carbon dioxide concentrations in the atmosphere has led to increased agricultural productivity.', 'The fact that carbon dioxide content has risen or fallen many times in the history of the earth suggests that there are some biological effects that can reverse the greenhouse effect.']