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Difficulty: HardUnstated Passage Assumptions

Passage:

In arid and semi-arid agricultural basins, progressive soil salinization poses a critical challenge to food security. As high-evaporation rates draw irrigation water upward through the soil profile, dissolved minerals—primarily sodium and chloride ions—concentrate in the upper root zone, inducing osmotic stress and disrupting nutrient uptake in standard crop varieties. Traditionally, farmers mitigate salinization by performing periodic soil leaching, a procedure requiring massive volumes of high-quality freshwater to flush accumulated salts beneath the root layer. However, severe regional water scarcity has rendered this practice increasingly untenable.

To address this limitation, agricultural biotechnologists recently formulated a composite bio-hydrogel soil amendment impregnated with specialized strains of halophilic rhizobacteria. When applied to the rhizosphere, the polymer matrix absorbs ambient soil moisture, while the encapsulated bacteria synthesize exopolysaccharides that bind free sodium ions, thereby restricting their bioavailability to plant roots. In a three-year controlled trial across several highly salinized test plots, crops cultivated in bio-hydrogel-amended soil exhibited a 40 percent decrease in tissue sodium concentration and produced a 25 percent higher crop yield than identical crops in untreated control plots receiving equal volumes of saline irrigation water.

Consequently, the developers argued that adopting this bio-hydrogel technology will allow commercial agricultural operations in arid basins to maintain long-term crop productivity on salinized land without relying on periodic freshwater soil-leaching operations.

Which of the following is an unstated assumption on which the developers' argument regarding long-term crop productivity depends?

  1. A
    Freshwater soil-leaching operations in arid basins consume a larger volume of regional water reserves than do all industrial applications combined.
  2. B
    Farmers in arid basins currently view soil salinization as a significantly more severe threat to agricultural output than the rising cost of irrigation water.
  3. The bio-hydrogel matrix and encapsulated bacteria do not gradually lose their sodium-binding efficacy or accumulate toxic salt loads in the root zone over successive cultivation cycles.Answer
  4. D
    The halophilic rhizobacteria encapsulated within the hydrogel matrix rely exclusively on organic root exudates from cultivated crops to sustain their metabolic functions.
  5. E
    Crops cultivated in salinized soils treated with the bio-hydrogel achieve market values equivalent to crops grown in non-saline soil.

Answer

The developers' argument assumes that the bio-hydrogel matrix and encapsulated bacteria will not gradually lose their sodium-binding efficacy or accumulate toxic salt loads in the root zone over successive cultivation cycles.
The correct answer identifies a condition necessary for the transition from short-term trial success (3 years) to continuous, long-term multi-season productivity without freshwater leaching. Applying the Negation Test demonstrates that if the hydrogel degrades or traps toxic levels of salt over time, the long-term cultivation strategy fails.

Step-by-Step Solution

1
Identify the main conclusion and key premises in the passage.
Conclusion: Adopting bio-hydrogel technology will allow long-term crop productivity on salinized land without relying on periodic freshwater soil leaching. Premise: A 3-year trial showed a 40% decrease in tissue sodium and a 25% yield increase in treated salinized plots.
Establishing the precise logical gap between short-term trial evidence and long-term operational feasibility is essential.
2
Formulate the unstated assumption linking the short-term trial results to the long-term claim.
The claim asserts 'long-term crop productivity' over multiple seasons without periodic flushing. Therefore, the mechanism preventing salt damage must continue working without degrading or creating toxic cumulative salt buildup over time.
An assumption is a necessary, unstated premise required for the conclusion to hold true.
3
Apply the Negation Test to verify the necessary assumption.
If the bio-hydrogel DOES lose its sodium-binding efficacy or DOES accumulate toxic salt loads over successive cycles, then continuous long-term crop productivity without periodic freshwater leaching becomes impossible, invalidating the developers' argument.
A valid GMAT assumption must cause the argument to collapse when logically negated.

Key Concept

Unstated Passage Assumptions and the Negation Test
Estimated Time:2m 30s
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Unstated Passage Assumptions

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