Self-study
Nature constantly adjusts the atmospheric carbon level. ██ ████████ ██ ███ █████ ██████ ███ ██████████ ██ ████ ████ █████ █████ ██████ ████ █████ ██ █████████ ████ ███ ███████ █████ ██████ █████████ █████ ████ ██████ ████ ██████ ████ ███ ███ ████ ███ ███████ █████ ██ ██████████ ███████ ████ ██ ███ ███████ █ ████████ ██ ███████████ ██████ ██████ ███ ██████████ ██ ████ ████ █████ █████ ██████ █████████ ███████████ ████ ███ ███████ █████ ██████ ████ █████ ███ ████ ████ ██████ ██ ██████ ████ ███ ███████ ███ ████ █████████████████ █████ ████ ███████ ██████ █████ ███ █████ ███████████ ██████ ██ █ █████████ ██████ ██ ██ ████ ████ █ █████████ ████████ █████ ████████ █████ █████ ███ ███ █████████████████ ██████ ██████████████ ████ ███████████ ██████ ███ ██████ ██████
Argument Breakdown
The stimulus tells us about atmospheric carbon levels. As context, we learn that environmentalists worry about fossil fuels leading to dangerous carbon levels. But although the author concedes that excessive atmospheric carbon would endanger human life, the author still concludes that the environmentalists have nothing to worry about. Why not? Because nature continually adjusts carbon levels. This is supported by a causal chain: more carbon warms the atmosphere, which leads to more evaporation, causing more rain, which helps to trap excess carbon in the oceans. So according to the author, this will all sort itself out.
Objective: Weaken
Our goal is to weaken the claim that carbon levels will not threaten human life. We also need to avoid attacking the premises though, which means our goal is to get in between the premises and the conclusion. In other words, we want to show that the conclusion could be false even though the premises are true.
So how is it possible that excessive atmospheric carbon could threaten human life even though nature has this regulatory mechanism? One possibility is that natural regulation has limits: if fossil fuels cause too much of a carbon excess, natural processes may not be able to fix the problem before it's too late, if at all. Or maybe fossil fuels interfere with natural regulation in some way. Whatever the correct answer choice targets, it will call into question the conclusion that environmentalists should just chill.
11.
Which one of the following, ██ █████ █████ ████ ██████ ███ ████████ ██ ███ ████████
a
Plant life cannot ███████ ███████ ███████████ ███████
While this is good to know, it has nothing to do with the dangers of excessive carbon. We're looking for a reason that too much carbon is dangerous, not too little.
b
It is not █████ ████ █████████ ██████ ██████ ██ ███ ██████████ ████ ████ █ ████████ ██████ ██ █████ █████
The stimulus
Even if the way in which excessive carbon would threaten humanity was important, (B) points in the wrong direction. It tells us that carbon might not be dangerous, but we need to show why it is dangerous.
c
Carbon is part ██ ███ ████████ █████████████ ████ █████ ███ █████ ████ ██████ ██ ███████ █████ █████
Like (A), this is a good reason why we need some carbon in our atmosphere. But that doesn't weaken the argument—to weaken, we have to show why too much carbon is dangerous, which (C) doesn't do.
d
Breathing by animals ████████ ██████ ██ █████ ██ ████ ██████ ██ ████ ███ ███████ ██ ██████ ██████
(D) might make us think that fossil fuels aren't that big of an issue. But there are two problems with that: first, that's the opposite of our goal, which is showing why excess carbon is dangerous. Second, we don't know enough about natural carbon regulation to know where the threshold is for a disaster. Maybe even a little more carbon would be enough.
e
The natural adjustment ████████ █████ ██████ ████ ████████ ██ ██████ ██████ ████ ████████████ ██ ███ ██████ █████ ██ ███ █████ █████
In other words, natural regulation happens too slowly to save humanity from the dangers of excess carbon. If nature heals in millions of years, great—but our species could easily be extinct by then. This shows a serious weakness in the argument, even if the premises are all true.