13. (D)
Question type: Strengthen the Conclusion
In this argument, a conclusion is justified through the use of an example. The argument can be strengthened if the connection between the conclusion and the example can be strengthened (i.e., if we can see that what happened with CFCs will happen with carbon dioxide levels).
(D) is the correct answer. It shows that, not only does this current situation _ carbon dioxide levels _ have the same characteristics as the CFC scenario, but also that it has these characteristics to a greater degree. This strengthens the conclusion.
(A) gives us more context for the discussion, but it doesn’t help us see that what happened with CFC’s will happen with carbon dioxide.
(B) could be correct if the degree of its impact was more significant. The fossil fuel producers are not saying that levels of carbon dioxide cannot be lowered _ but rather that they can’t be lowered to the point of halting global warming. Therefore, even if (B) was true, it wouldn’t go against the point made by the fossil fuel producers, and therefore doesn’t strengthen the conclusion.
(C) involves a comparison that is irrelevant to the argument at hand.
(E) is attractive, but finding a substitute is not what is specifically needed here. That substitute could be prohibitively expensive.