Hi Victoria,
This is a great question, for two important reasons: First, it’s an excellent idea to work on characterizing the nature of wrong answer choices, since this will help to strengthen your logical reasoning skills across the board. And second, since distinguishing between certainty and uncertainty is at the heart of every section of the LSAT, paying close attention to what makes an inference certain or uncertain is a brilliant studying strategy! This is especially true because unwarranted inferences are often featured in incorrect answer choices, on the hope that less savvy test-takers than you will go along with them in spite of their imperfections.
You’re right about the doubts you expressed regarding whether answer choice (A) tells us something we can justifiably infer from the facts of the stimulus. Since a valid inference is something that we know has to be true based on what we’ve been told in the stimulus, any potential inference must be held to a very high standard. In essence, we need to be
certain that our potential inference is the only possibility, and not just one thing among others that simply
might be true. Conversely, debunking an unwarranted or invalid inference is all about recognizing that there
are other possible truths that have not been completely ruled out, meaning that the inference we are considering is
not necessarily true, no matter how likely the stimulus may make it seem.
This is why we say that the answer choice in a Must Be True question—which needs to state a valid inference—has to pass the Fact Test,
i.e., be proven true by the facts of the stimulus.
Here, we are told by the stimulus that plankton cause the surface of the Earth to be cooler. As you suggested, this fact is not enough to prove (A)’s assertion that “the Earth would be far warmer than it is now if certain species of plankton became extinct.”
The statement made by (A) cannot be considered a valid inference because it is far too strong to be proven true by the stimulus for several reasons, one of which you already pointed out: The stimulus doesn’t give us any way to rule out the alternative possibility that “there may be some other species or mechanisms at play that may compensate if plankton were to go extinct.” The fact that plankton cool the Earth’s surface does not necessarily mean that, even if
all plankton became extinct, there would not be some alternate cause that would, as you said, step in and compensate for the lost cooling and thereby prevent the Earth’s surface from becoming warmer. Exactly as you suggested, we
do have to consider that a possibility, since the stimulus doesn’t rule it out.
Thus, although it would certainly be one possibility (and maybe even a likelihood) that the Earth’s surface would be warmer in the absence of plankton, the stimulus doesn’t give us any reason to believe that it is the
only possibility. Therefore (A) cannot rise to the level of something we know with certainty, or an assertion that
must be true, and thus cannot be a valid inference. (As you probably know by now from studying Mistaken Reversals and Mistaken Negations in conditional reasoning,
could be true does not a valid inference make!)
This would already give us enough reason to dismiss answer choice (A) if we were looking for a valid inference. But you would also want to be aware of some additional aspects of (A) that would prevent it from being a proper inference. First of all, even if the stimulus did tell us that there was no other cause of cooling that could make up for the loss of plankton, so that we
could infer that the Earth would be warmer without plankton, the stimulus would still leave us with no idea of
how much warmer, meaning that (A)’s use of “far warmer” would still not be proven true by the facts of the stimulus.
Finally, all we know from the stimulus is that
plankton in general cause the surface of the Earth to be cooler. Then (A) comes along and says that the Earth would be warmer if “
certain species of plankton” became extinct. But the stimulus gives us no way to differentiate among species of plankton and their roles in the cooling. Thus, we couldn’t possibly know what impact, if any, loss of
some species of plankton would have on the temperature of the Earth’s surface. Again, this lack of certainty gives us yet another reason why (A) is not proven true by the stimulus and thus couldn’t be considered a valid inference.
I hope this clarifies things!
Laura
