Logical reasoning PrepTest 121 · Section 4 · Question 3

Incorrect. The stimulus notes that a technique to tell the two types of ivory apart was developed, so this answer is somewhat contradicted by the stimulus.

Incorrect. There's no indication that there is another source of ivory, let alone that that source makes up the majority of the imported ivory.

Incorrect. The stimulus doesn't tie ivory harvesting to elephant population, nor does it discuss population at all, so this answer is out of scope.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
1989 - Ban on importation of ivory from elephants from Africa to US/Canada (still legal to import excavated mammoth ivory), but customs agents couldn't tell the difference
Right after ban passed - Sharp increase in "mammoth" ivory

Sometime later - Technique to distinguish elephant from mammoth ivory was discovered, and it became known that it was being used by customs officials
Right after technique publicized - Sharp decrease in "mammoth" ivory

Answer Anticipation:
This Must Be True stimulus isn't one where there are conditionals to chain together, or causality and comparisons that will allow inferences to be drawn. Instead, it's one where we're presented with a scenario and we need to take the totality of it into account in order to figure out an inference that can be made based on those details. As such, we should make sure we understand the timeline and the situation.

Here, there was a ban on something passed in 1989, and that was immediately followed by a "sharp increase" in the amount of "mammoth" ivory that was being imported. However, at this time, there was no way to distinguish the legal mammoth ivory from the illegal elephant ivory.

When such a method was developed and people learned that it was being used, the amount of "mammoth" ivory "dramatic[ally] decrease[d]".

What would explain the dramatic decrease once a technique for telling the legal from illegal ivory was publicized? We can't be 100% certain, but it sounds like a lot of people were lying about the source of the ivory they were sending to the US/Canada to get around the ban when they could get away with it, and then they stopped when they couldn't. And since this is a "most strongly supported" question, not 100% certain but still very certain is good enough.

Answer Explanation:
This answer highlights the (by far) most likely explanation for the sudden rise and fall of ivory labeled as being from ancient mammoth remains. Since the amount reported declined drastically once mammoth and elephant ivory could be told apart, it's very likely that people were lying about the source of the ivory to get around the band while they could get away with it.

Key Takeaway:
When a Must Be True stimulus presents a scenario that seems like it needs an explanation, the correct answer will bring up the most likely one. These questions tend to ask you to find an answer that's "most strongly supported" since the explanation isn't 100% certain, but there's almost always an explanation that makes much more sense in the context of all the details than any other potential explanation.

Incorrect. The stimulus notes a sharp increase in the amount of ivory identified as mammoth ivory, but that could have represented a higher percentage of the same total amount of ivory declared as mammoth ivory (and a matching lower percentage identified as elephant ivory).