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Difficulty: Very hardDistinguishing Inferences from Assumptions and Speculations

In a four-year ecological survey of Archipelago X, marine biologists monitored five deep-water reefs where commercial trawling was strictly restricted to depths shallower than 200 meters. Over the survey period, benthic invertebrate populations decreased by 35% across all five monitored reefs combined, while sea surface temperatures in Archipelago X remained within historical averages. On the two monitored reefs located within 10 kilometers of active deep-sea mineral dredging operations, benthic invertebrate populations decreased by 60%. On the remaining three monitored reefs, which were situated more than 50 kilometers from any dredging operations, benthic invertebrate populations decreased by 20%.

Which of the following can be properly inferred from the statements above?

  1. A
    The absolute number of individual benthic invertebrates lost on the two reefs situated near dredging operations exceeded the absolute number lost on the three reefs located farther from dredging operations.
  2. The percentage decrease in benthic invertebrate populations recorded on the monitored reefs located within 10 kilometers of dredging operations was greater than the percentage decrease recorded on those monitored reefs more than 50 kilometers away.Answer
  3. C
    Deep-sea mineral dredging operations were the direct primary cause of the benthic invertebrate population declines observed near Archipelago X.
  4. D
    Benthic invertebrate populations on the monitored reefs would not have declined if commercial trawling had been completely prohibited at all water depths.
  5. E
    Commercial trawling conducted at depths shallower than 200 meters contributed to the 20 percent population drop observed on reefs located more than 50 kilometers from dredging sites.

Answer

The percentage decrease in benthic invertebrate populations recorded on the monitored reefs located within 10 kilometers of dredging operations was greater than the percentage decrease recorded on those monitored reefs more than 50 kilometers away.
The passage explicitly states that benthic invertebrate populations decreased by 60% on the monitored reefs within 10 kilometers of dredging operations and by 20% on the monitored reefs more than 50 kilometers away. Because 60% is mathematically greater than 20%, it must be true that the percentage decrease was greater on the reefs located closer to dredging operations.

Step-by-Step Solution

1
Analyze the factual premises provided in the passage.
Identified explicit facts: 5 deep-water reefs monitored; trawling restricted < 200m; overall 35% decline; sea surface temps normal; 2 reefs within 10 km of dredging had a 60% decline; 3 reefs > 50 km from dredging had a 20% decline.
GMAT Critical Reasoning inference questions require evaluating statements solely based on what must be strictly true from the provided facts.
2
Distinguish strictly logical deductions from assumptions and causal speculations.
Recognized that comparing 60% to 20% yields a direct mathematical truth (60% > 20%), whereas asserting causation (dredging caused the drop) or absolute counts (total number of organisms) requires unstated facts.
Valid inferences cannot depend on unstated premises, external real-world knowledge, or speculative leaps.
3
Evaluate the answer choices against the strict deduction.
The statement comparing the percentage decreases on reefs near dredging (60%) versus reefs farther away (20%) MUST BE TRUE based solely on the explicit numbers given in the passage.
A 60% decline is strictly greater than a 20% decline, making this comparison a valid inference.

Key Concept

Distinguishing Inferences from Assumptions and Speculations
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Distinguishing Inferences from Assumptions and Speculations

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