The passage is arguing that environmentalists should not worry that burning fossil fuels may raise atmospheric carbon to a dangerous level because nature will continually adjust the carbon level thereby ensuring that a sustained increase which would threaten human life does not occur.
Answer choice B is incorrect because it is irrelevant to the argument. Neither the environmentalists nor the author discuss breathing excess carbon; they focus solely on the emissions which are released from the burning of fossil fuels.
Therefore, whether it is clear that breathing excess carbon in the atmosphere has a negative impact on human life or not has not bearing on the argument made by the author.
Hope this is helpful! Please let us know if you have any further questions.
