Self-paced
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.
The author also gives us a second, opposing causal chain to further illustrate how nature self-regulates. This second chain is concerned with insufficient (rather than excess) carbon, so it doesn't directly support the conclusion. We learn that lower carbon levels cause the atmosphere to lose heat, which leads to less evaporation, causing less rain, so more carbon accumulates in the atmosphere.
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.