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GMAT Club World Cup 2026 (Day 3): Biologists studying a species of

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AthaniX

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09 Jul 2026, 07:17

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A. Wear could come due to regular tunnel rubbing walls during transport. Weak
B. About different species
C. Could have multiple purposes, not concrete
D. Doesnt tell us WHY pebbles collected
E. Bingo
If Yes, tunnel reinforcement unlikely, supports grinding
If No, suggests usecase was to support against tunnel collapse

Hence Answer E distinguishes between the two hypothesises

Vijendra.D

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09 Jul 2026, 07:18

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Biologists observed pebbles moved from soil to nest
Ants needs seeds, Mandible cant crack seeds
Conclusion: Pebbles may fn as a grinding tool.

It also Strengthen nest tunnels

(E) Whether colonies of this species that nest in compact, stable soil - where the risk of tunnel collapse is not a concern - also transport pebbles into the nest interior

Yes this is the correct answer if they transport they need it for another purpose and if they stop transport they don't need seed cracking

Hence E is the answer

niteshmotwani

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09 Jul 2026, 07:24

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IMO - ANS is A
So here the says 2 reason one to break the hard seed and another reason is to support nest wall. and we need to evaluate in the sense both the above reasons are correct. or if we find something so that we can confidently say yes those hypothesis are correct.
In my opinion only Ans A make both reason correct by checking scratches / surface wear because it it breaks hard seeds it will get wear and if it supports the walls it still get some surface level wear.

Other option choice doesn't make sense to me so I go with option A.

B - just says about hard seed reason.
C - Irrelevant in my views
D & E - Also not make any sense for both hypothsis to be correct.
Thanks!

Bunuel

Biologists studying a species of harvester ant observed that workers regularly transport small pebbles from the soil surface into the nest interior. These ants subsist primarily on seeds, but their mandibles are too weak to crack the shells of the hardest seed varieties available in their habitat. Thus, the pebbles may function as a grinding tool, helping workers process seeds that their mandibles cannot crack. Alternatively, the pebbles may serve to reinforce the walls of the nest's tunnels, which are prone to collapse in the loose sandy soil the colony inhabits.

In order to help decide between the two hypotheses for the pebbles' function, which of the following would it be most useful to know?

(A) Whether pebbles recovered from the nest interior show greater surface wear than pebbles of similar composition collected from the surrounding habitat surface

(B) Whether other harvester ant species that are known to collect pebbles for structural reinforcement also inhabit regions where hard-shelled seeds make up a large proportion of the local food supply

(C) Whether the majority of the pebbles are deposited in the brood-rearing chambers rather than in the granaries or along the load-bearing sections of the tunnels

(D) Whether the hard-shelled seeds that this species cannot crack contain significantly more nutrients than the soft-shelled seeds their mandibles can process

(E) Whether colonies of this species that nest in compact, stable soil - where the risk of tunnel collapse is not a concern - also transport pebbles into the nest interior

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A. Irrelevant as more surface wear would support any of the 2 hypothesis. Grinding seeds would cause friction and wear but tightly packing the pebbles into walls to prevent structural collapse would also casue surface wear due ot the rubbing.
B. Irrelevant as other species behabior doesent tell us about the behavior and limitations of the current species being studied
C. Irrelevant as it doesent really choose between either of the hypothesis. Brood chambers are neither where food is processed nor where reinforcement is required. It groups both granaries and load bearing sections weirdly and doesent distinguish between the two properly
D. Irrelevant as nutritional value of seeds doesent explain whether pebbles are being used to crack the them open or if they are used for structural support
E. Relevant as this perfectly brings in a control group to evaluvate the pebble function. Basically if ants nesting in stable soil still bring in pebbles, it was due to the need for grinding seeds and if they dont bring in pebbles it was to prevent collapsing of their tunnels. This perfectly weakens either of the hypothesis based on the situation

E.

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09 Jul 2026, 07:34

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Passage analysis,
Observation: Ants transport pebbles.
Theories for the purpose of these pebblles.
Theory 1: Cracking hard shells
Theory 2: Reinforcing walls of the tunnel.

Question analysis, Most useful premise to reach the conclusion.

Answer choice analysis,

A. It is not the correct answer. Since, visible surface wears may not be there due to seed cracking. therefore, although presence of surface wear might infer something in this line. But absence of the surface wear definitively does not conclude anything.

B. This option is also not enough. Can not say anything definitively.

C. This is the correct answer. The location of the pebbles found in the nest can indicate the purpose of the pebbles.

D. This option is totally out of context. We are not bothered about the nutrition value of the seeds. We do know that their primary food is seeds. We do not have any idea about ants' preferences of seeds whether it is based on nutrition value.

E.This is totally opposite to the premise. Already mentioned in in the premise that their nest tunnels consists of loose sandy soil.

Bunuel

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09 Jul 2026, 07:44

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Pebbles function- grinding tool or reinforce walls which are prone to collapse in loose sandy soil.
A. Pebbles might suffer surface wear because of reinforcing walls or grinding or because of some other reason.......wrong
B. It talks about other harvestor ants......Wrong
C. Knowing where pebbles are stored won’t help much because they could be used for either purpose......Wrong
D. It talks about nutrition levels of seeds......Wrong
E. If same species which lives in stable soil, also carry pebbles then pebbles must be used for grinding and not for wall reinforcement. It strengthens one and weakens another........Correct

E

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09 Jul 2026, 07:48

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A Correct. If pebbles are used for grinding seeds, we'd expect wear marks from use. If they're just for structural support, wear would be minimal. This directly tests the grinding hypothesis.

B This is about correlation in other species, not direct evidence for this species' behavior.

C This could provide hints, but location alone isn't as definitive as physical evidence of use.

D This might tell us why they'd want to crack hard seeds, but it's irrelevant to test the hypothesis.

E This is a strong comparative test, but still indirect compared to wear evidence. A is better.

IMO A

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09 Jul 2026, 07:53

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A If inside pebbles are more worn than outside ones, that suggests they were used to grind hard seeds. If they look the same as outside pebbles, they were probably just moved for support. Correct answer.

B It only offers an indirect connection through other species, so it can't settle the question for the species we're actually studying.

C It gives a clue based on location, but location alone doesn't prove function (pebbles could be stored temporarily).

D It explains motivation, not the actual tool use.

E It falls short of proving the grinding hypothesis because it doesn't provide direct evidence like wear marks.

The answer is A

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Option- A irrelevant

Option- B irrelevant

Option- C irrelevant

Option- D irrelevant

Option- E is the correct answer, because there is no need to bolster the tunnel, then other obvious use is breaking the seed.

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First identify the 2 competing hypothesis : pebles are used to grind the seeds vs to provide structural support. Then identify an answer that MOST reinforces one of the two hypothesis, eliminating those that don't along the way:
A- Would give the first hypothesis strong support but we need to suppose that grinding pebbles damages the pebbles and that's not mentionned in the passage, so A is out.
B- No strong support for either hypothesis, B is out.
C - Out for same reason as B.
D - Out for same reason as B and C
E - Since the second hypothesis won't be valid if this information is true, this would support the first hypothesis. So E is our answer.