The conclusion of the argument is that the environmentalists should relax.
Why? "Nature will continually adjust the carbon level." Therefore, environmentalists shouldn't worry about fossil fuels raising atmospheric carbon to dangerous levels.
We are looking for the answer choice that weakens this argument.
Answer choice (D) states: "Breathing by animals releases almost 30 times as much carbon as does the burning of fossil fuels."
You're correct in pointing out that this could show that animals release exponentially more carbon than fossil fuels while no human life is threatened. This, however, almost strengthens the argument because it illustrates how inconsequential and irrelevant the burning of fossil fuels actually is. Thus, environmentalists, really have nothing to worry about!
However, we also do not know whether breathing by animals is also something to worry about. It could be true that breathing by animals is also something that threatens human life. We have been given no information on the effects of the carbon released by animals breathing. Therefore, it could be the case that both animals breathing and the burning of fossil fuels pose a threat to human life. So, this answer choice does not weaken our argument.
Answer choice (E) states: "The natural adjustment process, which occurs over millions of years, allows wide fluctuations in the carbon level in the short term."
Answer choice (E) tells us that the natural adjustment process occurs over millions of years and permits wide fluctuations in the carbon level in the short term. Thus, carbon released by the burning of fossil fuels, could actually raise the carbon level in the atmosphere for a sustained period of time, which would actually threaten human life. In this case, environmentalists actually do have something to worry about. Therefore, if answer choice (E) were true, it would weaken the argument.
Hope that clears things up! Please let us know if you have any other questions.
