Logical reasoning PrepTest 127 · Section 2 · Question 8

Incorrect. This answer is too weak in only talking about most boulders. Maybe the majority of boulders in question are sedimentary boulders, and volcanic boulders are generally moved much further. Since this answer doesn't provide enough information for us to be guaranteed that it applies to our volcanic rock, it's incorrect.

Incorrect. The closest source doesn't necessarily provide the most likely source of the volcanic rock, especially if there's no means established by which it could have moved north.

Incorrect. While there is a means by which a rock from the north could have moved to this area, and the explanation is that the glacier moved this boulder "hundreds of miles" and not just 50, there's no indication that the closest potential source of the boulder is the most likely source.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
This area has a lot of sedimentary rocks but only one volcanic. This area had south-moving glaciers. Thus, the volcanic rock was probably moved hundreds of miles by one of those.

Answer Anticipation:
This stimulus presents an outlier—a single example of a volcanic rock in a field of sedimentary ones. The conclusion then attempts to explain how it got there—southward-moving glaciers picked it up and moved it here from its origin hundreds of miles away. Whenever an argument attempts to explain a phenomenon, it can be undermined through two primary ways—either providing evidence that the given explanation is bad, or providing evidence that an alternative theory is good.

Two important things to note here about the logical force of the argument. First, it's about a single rock, so any answer choices about rocks would have to be strong to guarantee to apply to it. If an answer is about "most boulders," then it might not apply to this boulder, and thus that answer wouldn't be strong enough to weaken this argument. Second, the conclusion is that the explanation provided is "probably" true, so evidence would have to establish not that another explanation is possible but instead that it is probable before the conclusion would be weakened. That's a higher hurdle to clear, making it more likely the correct answer here will directly undermine the author's theory instead of providing evidence for a competing one.

Answer Explanation:
This answer directly undermines the explanation presented in the stimulus. That explanation featured southward-moving glaciers bringing the boulder to the area, so they would have brought it from the north. If there are no sources of volcanic rock up north, then it almost certainly wasn't moved from there by those glaciers, and the conclusion is undermined.

Key Takeaway:
This question is notoriously difficult, but it can teach you a lot about the intersection of logical force and Weaken questions. Paying attention to the strength of the premises and conclusion can tell you a lot about how strong the answer has to be here. In general, Weaken (and Strengthen) questions are interesting in that the weaker the argument, the stronger the right answer must be, and vice versa. Here, the premises/conclusion were relatively weak (one rock; "probably" true conclusion), so the correct answer was likely to be strong (as we saw here).

Incorrect. We know that this boulder is of volcanic origins and is in the area, so the lack of other, similar boulders in the area might present a mystery, but it still has an explanation.