Logical Reasoning

PT151 · S2 · Q18 In grasslands near the Namib Desert

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In grasslands near the Namib Desert there are “fairy circles”—large, circular patches that are entirely devoid of vegetation.

Conclusion (hypothesis)

The burrowing activities of sand termites cause fairy circles to form.

Evidence (since)

Sand termite colonies were found in every fairy circle the scientists investigated.

Evaluate

Whenever we see "hypothesis / explanation" in the question stem, we know we'll be reading an Explain Curious Fact style argument.

Here, the CURIOUS FACT was, , and the author's causal explanation is, "They're caused by termites burrowing into the soil".

We can strengthen by Ruling Out an Alternate Explanation or by Boosting the Plausibility of the author's explanation.

On Strengthen questions, it's 2.5x more common to boost plausibility.

Goal

Let's rule out an alternate explanation, e.g. , or let's make it more plausible that they are formed from burrowing termites.

18.

Which one of the following, if true, most supports the scientists' hypothesis?

Correct
Dying grass plants within newly
Why this is right
This boost the plausibility of the burrowing hypothesis. To burrow into the ground is to dig yourself into the dirt. Doing so near a plant would potentially damage the roots (the underground portion of a plant) but wouldn't affect the top of the plant (conversely, a grazing goat would affect the top of the plant but wouldn't affect the roots). If grass plants in newly forming fairy circles are damaged only at the roots, not at the leaves, stems, or above-ground parts, that points to an underground cause. The damage pattern matches the proposed mechanism of burrowing termites. So this is corroborating the author's story. We're in the grasslands, termites show up and burrow into the dirt, damaging the roots of the nearby plants. Those plants die and we now have a new fairy circle, devoid of vegetation.
Skill tested: Strengthen · how this choice captures the argument's function is the move to repeat next time.
52% picked this
The grasses that grow around
Opposite
This answer suggests that the grasses in the grasslands where we find fairy circles are pretty hearty and wouldn't die from a lack of water. So in that sense it undermines an alternate explanation of "fairy circles form when there's a lack of water", because it sounds like if there was once vegetation in the fair circles, it wouldn't die out from lack of water. But ultimately ruling out one possible explanation for fairy circles isn't as impactful as the corroborating evidence in (A). This answer helps us think, "Fairy circles aren't caused by lack of water", but (A) actively helps us think, "Fairy circles are caused by burrowing termites". We already had the termites at the scene of the crime, and when we look at the victim (the dying grass plants), they seem to have been attacked by the type of murder weapon (burrowing) that the termites are holding.
10% picked this
The soil in fairy circles
No Impact
Just like (B), this answer undermines the alternate explanation that "fairy circles form when there's a lack of water", because it sounds like there's plenty of water where the fairy circles form. But ultimately ruling out one possible explanation for fairy circles isn't as impactful as the corroborating evidence in (A). This answer helps us think, "Fairy circles aren't caused by lack of water", but (A) actively helps us think, "Fairy circles are caused by burrowing termites". We already had the termites at the scene of the crime, and when we look at the victim (the dying grass plants), they seem to have been attacked by the type of murder weapon (burrowing) that the termites are holding.
3% picked this
Fairy circles tend to form
No Impact
The fact that fairy circles cluster together doesn't seem to have a clear impact on making the termite burrowing hypothesis more / less plausible. Would we expect burrowing termites to cluster together in areas with other groups of burrowing termites? I don't really have an expectation there. And if we were considering alternate hypotheses like "they're formed by lack of water", that sort of hypothesis could easily be compatible with this "fairy circles" are clustered together idea, because it's probably common for an entire region to be low on water, not just one circle. So there's just no clear connection from clustered circles to the author's or any alternate hypothesis.
6% picked this
Species of animals that feed
No Impact
We already know that termites are on the scene, when we find fairy circles. So it's not surprising that animals that eat termites show up for a snack. If anything, this starts to suggest an alternate explanation that the vegetation in fairy circles gets trampled by animals that show up to eat termites (not because of the termites' burrowing).
29% picked this

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