Alright, let's see if we can figure this one out. The question is #25 in the first Logical Reasoning section (section 2 of the test version that I have), and is a Cannot Be True question. Well, technically it's phrased as "could be true EXCEPT," but that's the same thing as saying the correct answer cannot be true
The stimulus tells us that for small/medium sized marine mammals, the longer the animal can remain underwater, the deeper it can dive, and vice versa. As soon as I read that I realized a large part of the difficulty here can be removed: just treat dive depth and submerged time as the same thing! That is, instead of multiple relationships between the mammals listed, reduce it down to just single truths, where dive depth = submerged time. This is possible because, as we'll see, it's only about relative comparisons, not exact amounts or perfect one-to-one comparisons.
I'll show you what I mean.
We get a comparison of four mammals' diving depths and submerged times:
Dolphins can dive deeper than northern fur seals can
Elephant steals can stay under longer than Weddell seals can
We can infer two things from this:
Dolphins can stay submerged longer than northern fur seals can
Elephant seals can dive deeper than Weddell seals can
So by basically treating depth and time as the same, two relationships are known:
Dolphins (D) > northern fur seals (NFS)
Elephant seals (ES) > Weddell seals (WS)
And that's it!
But we need to be careful. Any attempt to cross-compare between the two groups directly—comparing dolphins to elephant seals, say—would be trouble. Not false, necessarily, but definitely impossible to conclude with certainty. The only thing you could do across groups would be to create impossible chains, like saying elephant seals can dive deeper than dolphins (so stay under longer than dolphins, ES > D) but not stay submerged as long as northern fur seals (NFS > ES): if they can stay under longer than dolphins, they MUST be able to stay under longer than northern fur seals. If ES > D, then ES must be > NFS. That type of relationship is the only way to go across groups and know anything.
So what I would expect of many, and possibly all, of the four wrong answers (the four that all could be true) is a type of unknowable cross-comparison. What I expect of the correct answer (the one that cannot be true) is either a reversal of one of those two inferences I listed above (doubtful though as that's pretty easy to spot), or a cross-comparison that creates an impossible ordering like the example I gave above (ES > D; NFS > ES).
Let's go through 'em:
(A) gives us ES > D > WS. This answer is out because that's a totally possible dive depth order (we know elephant seals can outdive Weddells, and it's possible dolphins are in between). Comparing dolphins to the other group with certainty is impossible in this case.
(B) gives us D > WS > NFS. This answer is out for the same reason as (A): we know dolphins > northern fur seals, and since Weddell seals are in the other group inserting WS in the middle here is entirely possible.
(C) gives us WS > D and NFS. Once again, we've got a cross-comparison that we can rule out with certainty. We know D > NFS, which this answer still allows, and putting WS in front of both is entirely possible.
(D) is correct. It gives us two chains: NFS > ES, and WS > D. So why can't that happen? Imagine the first part is true, and NFS > ES. That creates a long chain with all four mammals: D > NFS > ES > WS. But that directly contradicts the second part of answer choice (D), which says that WS > D! So this won't work, and we can say with confidence that this is the correct answer.
Similarly, (D) fails if you start with the second part, WS > D. That too creates a long chain: ES > WS > D > NFS. But that directly contradicts the first part of answer choice (D), where NFS > ES. Clearly then those two options cannot both occur, and this answer cannot be true.
(E) gives us ES > NFS > WS. This is just like answer choice (A), except instead of inserting dolphins between elephant seals and Weddell seals, here we put northern fur seals in the middle. But that's totally fine, just as (A) was.
So that's my breakdown of this question. It's a fairly long and tricky one, but hopefully seeing it the way I laid it out above helps to remove some of the mystery.
Let me know if that's clear, and certainly if you still have questions feel free to chime in!
Jon
