CAT Critical Reasoning Practice question with Solution 63

Nature constantly adjusts the atmospheric carbon level. An increase in the level causes the atmosphere to hold more heat, which causes more water to evaporate from the oceans, which causes increased rain. Rain washes some carbon from the air into the oceans, where it eventually becomes part of the seabed. A decrease in atmospheric carbon causes the atmosphere to hold less heat, which causes decreased evaporation from the oceans, which causes less rain, and thus less carbon is washed into the oceans. Yet some environmentalists worry that burning fossil fuels may raise atmospheric carbon to a dangerous level. It is true that a sustained increase would threaten human life. But the environmentalists should relax—nature will continually adjust the carbon level.

Which one of the following, if true, would most weaken the argument in the passage?

[A]. Plant life cannot survive without atmospheric carbon.
[B]. It is not clear that breathing excess carbon in the atmosphere will have a negative effect on human life.
[C]. Carbon is part of the chemical “blanket” that keeps the Earth warm enough to sustain human life.
[D]. Breathing by animals releases almost 30 times as much carbon as does the burning of fossil fuels.
[E]. The natural adjustment process, which occurs over millions of years, allows wide fluctuations in the carbon level in the short term.
Answer: E

The author concludes that environmentalists are wrong to worry about increased carbon levels due to the burning of fossil fuels, because nature will continually adjust the carbon level. However, if, as choice (E) states, the adjustment process works very slowly, allowing wide short-term fluctuations in the carbon level, then it’s conceivable that dangerous or even lethal levels of carbon can build up, thus severely weakening the author's argument: the environmentalists would be right to worry.

(A), (C) The author doesn’t debate the necessity of carbon, she merely argues that nature takes care of the carbon level by itself, and that the environmentalists should chill.

(B) Some may feel that this weakens the author’s concession in the last sentence, but even if it did, that sentence is basically a tangential point that has little to do with the logic of the argument, which deals with nature’s regulation of the carbon level. However, this probably doesn’t even affect the author’s concession, because that statement never said or implied that the threat to humans of increased carbon levels would come from breathing it in; it would likely be more indirect, along the lines of the processes described.

(D) This is an irrelevant comparison that has no bearing on the logic of the argument.

Recognize filler material (like the author’s concession that a sustained increase in the carbon level would threaten human life) that plays no real part in the overall logic of the argument. There’s usually a wrong choice that keys off of such irrelevant info.

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