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Difficulty: MediumNecessary Assumption

Agricultural Historian: Soil samples from ancient agricultural terraces in the arid Zafaran basin contain significantly higher concentrations of arbuscular mycorrhizal fungal spores than soil samples from adjacent uncultivated hillsides. These fungi form symbiotic associations with plant roots, substantially increasing drought tolerance and nutrient uptake in grains. Because surviving historical records from that era describe complex agricultural soil treatments, the farmers in this basin must have deliberately inoculated their terrace soils with fungal spores to enhance grain harvests.

Which one of the following is an assumption required by the agricultural historian's argument?

  1. A
    Deliberate soil inoculation was the primary method used by ancient Zafaran farmers to mitigate crop failure during severe droughts.
  2. B
    Grains cultivated on the terraces could not have survived in the arid Zafaran basin without forming symbiotic associations with mycorrhizal fungi.
  3. C
    The historical records describing soil treatments explicitly mention the biological properties of microscopic soil organisms.
  4. The elevated spore concentrations in the terrace soil were not entirely the natural result of cultivating grains that fostered indigenous fungal growth.Answer
  5. E
    The adjacent uncultivated hillsides were never subjected to natural runoff containing fungal spores from the agricultural terraces.

Answer

The argument depends on assuming that the elevated concentration of fungal spores in the terrace soil was not merely a natural by-product of cultivating grains that attracted indigenous fungi.
The conclusion claims that the presence of high spore concentrations in terrace soils resulted from deliberate human inoculation. For this inference to hold, the elevated spore density cannot be explained away as an automatic, passive ecological byproduct of growing crops on that soil. The correct statement eliminates this competing explanation, making it an indispensable defender assumption.

Step-by-Step Solution

1
Deconstruct the argument into premises and conclusion
Premises: (1) Ancient terrace soil has higher fungal spore concentrations than adjacent uncultivated soil. (2) Mycorrhizal fungi increase grain drought tolerance and nutrient uptake. (3) Historical records mention complex soil treatments. Conclusion: Ancient farmers deliberately inoculated their terrace soils with fungal spores.
Isolating the conclusion identifies the logical leap between observed physical evidence (high spore counts) and human intent (deliberate inoculation).
2
Identify the unstated logical gap / potential alternative explanation
The author presumes that human intervention caused the elevated spore levels rather than a natural biological response triggered simply by planting crops.
A necessary assumption must defend the argument against plausible alternative causes that would explain the premise without requiring the conclusion.
3
Apply the Negation Test to confirm necessity
Negating the statement gives: 'The elevated spore concentrations were entirely the natural result of cultivating grains that fostered indigenous fungal growth.' If true, this eliminates the evidence of intentional inoculation, causing the conclusion to fail.
A statement is an unstated necessary assumption if and only if its logical negation fatally undermines the argument's conclusion.

Key Concept

Necessary Assumption Identification and Defender Assumptions
Estimated Time:1m 20s
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Topics

Necessary Assumption

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