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

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Maesha

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08 Jul 2026, 10:58

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option A isn't directly related to the passage . The change on the surface of pebble can also be caused by various reasons , such as natural phenomena.

option B : Just one observational study cannot draw the conclusion that the habitual behavior of all species of ants is same across the world . A cross sectional study is needed to check this assumption.

option c : it is not sure whether the ant keeps the pebbles in depository or chamber or load bearing sections . The ways to deploy the pebbles can vary among species.

option d : even if the non crackable hard shelled seeds are full of nutrition that doesnt mean these are mandatory to crack pebbles .

option E : this is possible ,if species living in safe soil also carry pebble that means these pebble are used for other concern not just to protect their shells.

so correct ans is option E.

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The correct answer is E.

The other options are less effective and fail to directly distinguish between the seed grinding and structural reinforcement hypotheses.
Option A mentions the surface area which can be valid in both scenarios.
Options B, C, D rely on comparison with external species, an irrelevant location, or nutritional value rather than focusing on the behavioral evidence.

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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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I chose C although i now feel that it could even be E.

1. A- In both uses of pebbles as a protective barrier and in grinding seeds, they will suffer wear and tear. Hence this does not help in choosing either hypothesis. Exclude

2. B - No relevance to other ant species as it still does not give a clear winner between the two hypothesis for the species in question. Exclude

3. C - I selected C thinking that brood reading chambers would imply usage of pebbles for food purposes i.e grindinh hence Hypothesis 1 AND load bearing sections implying protective barriers hence Hypothesis 2. As they are both giving clear answers i chose C

4. D - Nutrient content does not answer either of the hypothesis. Exclude

5. E - Probably the right answer here. These species do not require the barrier (Hypothesis 2) and if they still bring pebbles that means they use it for grinding (Hypothesis 2). If not, Hypothesis 1 stands true

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Statement 1 - Pebble being used as a grinding tool to help with food.

Statement 2 - Pebbles are used to reinforce nest when build in loose sandy soil

To find the exact purpose, we need to find the case where one is true while the other is false

From the option for E, it talks about if the pebbles are found in the interior even in stable tunnel environment. If they are found, it means its not only for when tunnels are unstable, but also when stable. Implying the pebbles are used no matter what the condition of the tunnel environment is

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08 Jul 2026, 12:37

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The 2 hypothesis in the question are : Ants use pebbles to crack the shells of the hardest seeds Or Ants use pebbles to serve as nesting wall in loose soil.
Option A - Whether pebbles recovered from the nest interior show greater wear than pebbles of similar composition collected from the surrounding habitat surface = Incorrect since wear and tear can happen when the pebbles are used for both cracking of shells of seeds or nesting wall and hence determination not solely possible
Option 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 - this option is irrelevant - Even if we get to know if other ants inhabit such region, how will we link it
Option C - Whether the majority of the pebbles are deposited in the brood-rearing chambers rather than in the granaries of along the load bearing sections of the tunnels - This is a little confusing - Although we get to know where the majority of pebbles are collected - it might not indicate actual usage - hence not selecting this option
Option D - Whether the hard-shelled seeds that this species cannot crack contain significantly more nutrients than the soft-shelled seeds their mandibles can process - Totally out of scope - the question does not talk about nutrients.
Option 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 - Correct - this will help prove any one hypothesis mentioned in the beginnning. Hence this option is correct

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08 Jul 2026, 12:41

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Bunuel

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Hypothesis 1 -> pebbles grind seeds (food)
Hypothesis 2 -> pebbles reinforce walls (building)

why option C works because location reveals purpose, and granaries (food storage) proves seed grinding, tunnels (walls) proves reinforcement, Brood chambers (babies) -> it disproves both.

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08 Jul 2026, 13:10

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IMO - it should be option A. I was confused between A and E, though.

I chose A because it looks at how pebbles were actually used. If they were used for grinding, they should show more surface wear than similar pebbles found nearby. This helps distinguish between 2 hypotheses that the argument talks about.

E - Feels relevant, but it does not clearly separate the 2 hypotheses. Even if the ants in stable soil collect pebbles, they could be using them for another purpose.

Looking at the remaining options below:
(B) Too may varibables and does not explain how species use pebbles.
(C) Does not address the difference between the two hypotheses.
(D) Irrelevant - Nutritional value of seeds has nothing to do with the use of pebbles.

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A. Not the best
B. Not useful
C. Dont resolve question
D. Not relavant
E. If the ants live in stable soil and still carry pebbles than they must use them for opening nuts. If the stop carrying pebbles than they use them for structure of nest

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Now, to decide between the two hypotheses, we need to choose the option that would differentiate the usage of pebble and support / oppose one of the hypotheses.
(A) Hypotheses are not about surface wear of pebbles.
(B) We don't want to compare between ant species.
(C) Location of pebbles won't tell anything concrete.
(D) Nothing to do with the nutrients present.
(E) Correct option. By knowing whether pebbles are transported by the ants in stable soil, we can confirm between the two hypotheses.

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08 Jul 2026, 13:43

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A--- It talks about wear and tear basically which wont help us in evaluation. Drop
B---This doesnt help us evaluate the agreement at all, it talks about other harvester ants. Drop
C---The location of the pebble wont help us, notice the word majority. Drop
D--- This talks about nutrients which is not relevant to the security of the ants home. Drop
E---This is correct, notice it talks about structural integrity and helps understand if they use pebbles for protection or for cracking food purpose.

Hence E is our best option

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I think E) because if their tunnel is not at risk to collapse, then it doesn't make sense to say that they need the pebbles for construction

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A) Letter A can't be correct since rocks are harder than seed, and moving their position to the interior probably doesn't affect surface wear either.
B) Knowing what other species do doesn't mean I'll know for sure what this species does.
C) This doesn't help. It would be more beneficial to know whether it was found in the load-bearing sections or in the granaries.
D) This doesn't directly answer the hypothesis, since it is talking about nutrients, not the use of pebbles.
E) This can give us a clue, at least to know whether the second hypothesis is true or not. So E is the correct answer.

Bunuel

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08 Jul 2026, 16:42

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There are two hypothesis here:
1. Pebbles help grind hard seeds.
2. Pebbles reinforce the walls of nest tunnels.

Evaluating the options:

(A) Supports the grinding hypothesis but not strongly about the structural hypothesis. (Seems close but incorrect)
(B) Talks about other ant species which has nothing to do with the current species we are evaluating. (Incorrect)
(C) Too many foreign terms for me. Eliminating due to less relevancy. (Incorrect).
(D) Hard seeds being nutritious doesnt connect the point that pebbles are actually used to grind. (Incorrect).
(E) Provides support in evaluating if ants building colonies in stable soil still collect seeds in stable soil will target both the hypothesis. (Correct)

IMO Ans is E.

Bunuel

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A - wear and tear can be caused by the soil around it or by grinding, so surface area evaluation isn't helpful in determining what caused it to become smooth
B - Doesn't answer our question
C - almost fell for this, but brood isn't talked about, if they just compared granaries to rear ends, this answer would help
D - nutrients or nutrition isn't the point, NA
E - Yes! If colonies of this same species is still taking the pebbles underneath then its clear, the purpose is for grinding only.

Answer - E

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08 Jul 2026, 17:06

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Statement 1 tells us that if there is surface wear then the pebbles have been used for grinding thus it proves one hypothesis

Bunuel

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08 Jul 2026, 19:07

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Went with C) but was very confused between C) and E)
We want to know if pebbles were used for walls or for cracking seeds. So, I will do a Yes/No test on each

A) if pebbles show more wear or less wear doesn't matter since in any case they may have been used for either purpose - ELIMINATE
B) This is going too far. We don't care about regions where seeds make up 'large' prop of food supply. Whether or not ants inhabit such regions doesn't tell us what pebbles were used for. ELIMINATE
C) assuming it's talking about 2 regions - one related to food, and other related to walls (load bearing) - if it's found in granaries then used for food, if in load section, then wallsKEEP
D) info about nutrients tell us nothing - ELIMINATE
E) this looks good so having a hard time in eliminating it. Just thinking this is only talking about the ants that are in compact, stable soil which may be a minority so may not really tell us anything solid. ELIMINATING (though not 100% sure)

ANS C)

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08 Jul 2026, 19:36

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There are 2 hypotheses for the pebbles; (a) grinding tool and (b) reinforcing the nest tunnels.
Let's test the hypotheses using Yes/No to support
(A) Interior pebbles show greater wear than the habitat surface pebbles?
Yes - subsist primarily on seeds. > support (a), No - pebbles sitting in the surface > support (b) [Answer]
(B) Other pebbles for structural reinforcing ants also inhabit, where hard-shelled seeds are common?
Yes - Did not tell us why the ants carry pebbles. > not supporting either of the hypotheses
No - Did not tell us the function either > not supporting. Eliminate.
(C) Most pebbles live in brood-rearing chambers?
Yes - brood-chamber does not fit in grinding or reinforcement > does not support
No - Could be granaries (support a) or load-bearing section (support b) > the spliting is ambiguous. Eliminate.
(D) Supports only (a). Eliminate.
(E) Weakens (a) and (b) in different proportion. Eliminate.

Thus, the answer is A.

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08 Jul 2026, 19:39

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We need an option that helps us finalise one of the below two reasons.
1) ROcks used for structural stability
2) Rocks used to grind the hard shells of seed.

Option-1 : Seems close but NO. Because if the surface of the pebbles inside is smooth, it need not be necessarily because of grinding. It can be because of other reasons like pebbles rubbing against each other when placed for structural integrity or because of sand carried by ants rubbing against these rocks etc.

opt-2: Getting data about other harvester ants cannot really clarify the hypothesis here.
opt-3: If they are not in brood rearing chambers they can be in granaries or along load bearing sections. Hence this options also doesn't clarify the paradox.
opt-4 : Nutrient content is not the issue here.
opt-5: Yes,
if the answer to this option is Yes, then they are carrying pebbles for purpose other than structural strength.
If the answer is no, then the pebbles carried are only for structural strength.

Opt-5 is the answer

Bunuel

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08 Jul 2026, 19:59

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Correct answer: E
Because we controlled one cause to check whether other causes lead to the effect.

If 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, it means that ants may be using it for grinding purposes.
If not, ants may be using the pebbles for reinforcing purposes.

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08 Jul 2026, 20:24

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Option E clearly removes one hypothesis (Support to Falling Tunnel), which shows that if they are still transporting pebbles despite strong support to the tunnel, it must be for breaking seeds, and if they are not transporting here, it would be for breaking seeds when colonies are built in sand.