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Difficulty: Very hardNecessary Assumption

Agronomist: For decades, the persistently low crop yields in the Xanthus Valley were universally attributed to the high concentration of free aluminum ions in the soil, which are highly toxic to most commercial crops. To resolve this, farmers recently treated the valley's soil with a novel synthetic chelating agent that permanently neutralizes all free aluminum ions, rendering them completely biologically inert. However, subsequent harvests in the treated fields have shown no increase in crop yields. Therefore, it is evident that the high concentration of free aluminum ions was not the primary factor limiting crop yields in the valley.

Which one of the following is an assumption on which the agronomist's argument depends?

  1. A
    The neutralization of any single factor that limits crop yields in a given region will invariably result in a significant increase in subsequent harvests.
  2. B
    Prior to the application of the chelating agent, free aluminum ions were the only factor in the Xanthus Valley soil capable of severely limiting crop yields.
  3. The synthetic chelating agent does not itself possess properties that inhibit crop yields to a degree that would offset any potential gains from neutralizing the free aluminum ions.Answer
  4. D
    The commercial crop varieties typically planted in the Xanthus Valley are not naturally resistant to the toxic effects of free aluminum ions.
  5. E
    The persistently low crop yields in the Xanthus Valley will not improve until the actual primary limiting factor is identified and successfully neutralized.

Answer

The argument must assume that the synthetic chelating agent does not itself possess properties that inhibit crop yields to a degree that would offset any potential gains from neutralizing the free aluminum ions.
The argument observes that neutralizing aluminum didn't increase yields, and concludes aluminum wasn't the primary problem. This reasoning has a massive blind spot: what if the chemical used to neutralize the aluminum damaged the crops just as much as the aluminum did? In that scenario, aluminum could absolutely have been the primary historical problem, but the farmers replaced one problem with another, resulting in zero net gain. For the agronomist's conclusion to survive, they MUST assume that the chelating agent didn't introduce a new limitation that masked the benefits. This is a classic 'Defender' assumption.

Step-by-Step Solution

1
Analyze the stimulus to identify the conclusion and the premises.
Conclusion: High concentration of free aluminum ions was not the primary factor limiting crop yields. Premise: A chelating agent neutralized all aluminum, but subsequent harvests showed no increase in yields.
Understanding the structural relationship between the evidence and the conclusion is the first step in finding the logical gap.
2
Identify the logical gap in the agronomist's reasoning.
The agronomist assumes that a lack of yield increase strictly means the neutralized factor (aluminum) wasn't the main problem. This overlooks the possibility that the 'cure' might be causing its own problems.
Arguments based on experiments or interventions often fail to account for unintended side effects of the intervention itself.
3
Apply the Negation Test to the correct option.
Negating the correct option yields: 'The chelating agent DOES possess properties that inhibit yields to a degree that offsets the gains of neutralizing aluminum.' If true, yields wouldn't increase EVEN IF aluminum was the primary culprit. This completely destroys the conclusion.
The Negation Test is the definitive way to prove whether an assumption is strictly necessary for the argument to hold.

Key Concept

Defender Necessary Assumption (Ruling out alternative explanations for evidence)
Estimated Time:2m 15s
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Topics

Necessary Assumption

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