Plankton generally thrive in areas of the ocean with sufficient concentrations of certain nitrogen compounds near the surface where plankton...
GMAT Critical Reasoning : (CR) Questions
Plankton generally thrive in areas of the ocean with sufficient concentrations of certain nitrogen compounds near the surface where plankton live. Nevertheless, some areas, though rich in these nitrogen compounds, have few plankton. These areas have particularly low concentrations of iron, and oceanographers hypothesize that this shortage of iron prevents plankton from thriving. However, an experimental release of iron compounds into one such area failed to produce a thriving plankton population, even though local iron concentrations increased immediately.
Which of the following, if true, argues most strongly against concluding, on the basis of the information above, that the oceanographer's hypothesis is false?
Passage Analysis:
| Text from Passage | Analysis |
| Plankton generally thrive in areas of the ocean with sufficient concentrations of certain nitrogen compounds near the surface where plankton live. |
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| Nevertheless, some areas, though rich in these nitrogen compounds, have few plankton. |
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| These areas have particularly low concentrations of iron, and oceanographers hypothesize that this shortage of iron prevents plankton from thriving. |
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| However, an experimental release of iron compounds into one such area failed to produce a thriving plankton population, even though local iron concentrations increased immediately. |
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Argument Flow:
The passage starts by establishing a general rule about plankton needing nitrogen, then introduces a puzzling exception where nitrogen-rich areas have few plankton. Oceanographers propose iron deficiency as the explanation, but an experiment that added iron failed to increase plankton populations, which seems to challenge their hypothesis.
Main Conclusion:
There isn't a clear main conclusion stated - rather, the passage presents a scientific puzzle where experimental evidence appears to contradict the oceanographers' iron deficiency hypothesis.
Logical Structure:
This follows a problem-hypothesis-counterevidence structure: We have a puzzle (nitrogen-rich areas with few plankton), scientists propose an explanation (iron deficiency), but experimental evidence doesn't support this explanation, leaving us uncertain about whether the hypothesis is correct.
Prethinking:
Question type:
Strengthen - We need to find information that supports the oceanographers' hypothesis despite the failed experiment. This is essentially defending their theory against what looks like contradictory evidence.
Precision of Claims
The key claims involve specific conditions: iron deficiency prevents plankton thriving, and the experiment added iron compounds with immediate concentration increases but no population growth. We need to be precise about timing, conditions, and what constitutes 'thriving'.
Strategy
Since the experiment seems to contradict the oceanographers' hypothesis, we need to find reasons why the experiment might not actually disprove their theory. We should look for scenarios that explain why adding iron didn't work this time, while still keeping the iron hypothesis valid. Think about timing issues, other missing factors, or problems with the experimental conditions.
This tells us that not all nitrogen compounds are nutrients for plankton. However, the passage already establishes that plankton thrive in areas with 'sufficient concentrations of certain nitrogen compounds' - implying the right types are present. This doesn't explain why adding iron failed to help plankton populations, so it doesn't defend the oceanographers' hypothesis.
This actually weakens the oceanographers' hypothesis rather than strengthening it. If some areas have abundant plankton despite low iron concentrations, this suggests iron deficiency might not be the key factor preventing plankton from thriving. This works against the hypothesis we're trying to defend.
This states that releasing iron compounds didn't increase nitrogen supply in the area. But since the passage tells us these areas are already 'rich in nitrogen compounds,' the problem isn't nitrogen availability. According to the hypothesis, iron is the missing factor, so this information is irrelevant to explaining why the experiment failed.
This provides a perfect explanation for why the experiment failed without disproving the hypothesis. If ocean currents moved the iron-rich water away from the surface within days of the release, then the plankton never actually had sustained access to both nitrogen and iron simultaneously. The experiment didn't properly test the hypothesis because the experimental conditions were disrupted. This strongly defends the oceanographers' theory.
This tells us the released iron compounds occur naturally where plankton thrive, which suggests the right type of iron was used. However, this doesn't explain why the experiment failed to produce results. While it supports the general idea that iron helps plankton, it doesn't address the specific problem of why this experiment didn't work despite using apparently correct iron compounds.