A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Evaluate

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A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants. Agronomists studying the growth of this herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert. Hence, the herb's high histidine production must be the key feature that allows it to grow in metal-rich soils.

In evaluating the argument, it would be most important to determine which of the following?

Whether the herb can thrive in soil that does not have high concentrations of the toxic metals

Whether others of the closely related group of plants also produce histidine in large quantities

Whether the herb's high level of histidine production is associated with an unusually low level of production of some other amino acid

Whether growing the herb in soil with high concentrations of the metals will, over time, reduce their concentrations in the soil

Whether the concentration of histidine in the growing herb declines as the plant approaches maturity

Solution

Passage Analysis:

Text from Passage	Analysis
A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants.	What it says: This herb grows well in metal-rich soil that kills most other plants What it does: Sets up the puzzle - how can this herb survive where others can't? What it is: Background fact/observation
Agronomists studying the growth of this herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert.	What it says: Scientists found the herb makes lots of histidine, and histidine neutralizes toxic metals in lab tests What it does: Provides key evidence that could explain the herb's survival ability What it is: Research finding Visualization: Lab test: Toxic metals + Histidine → Metals become harmless
Hence, the herb's high histidine production must be the key feature that allows it to grow in metal-rich soils.	What it says: The author concludes that histidine production is what lets the herb survive in toxic soil What it does: Makes the logical leap from lab results to real-world explanation What it is: Author's conclusion

Argument Flow:

The argument starts with an observation about an unusual herb that can survive in toxic soil, then presents lab evidence showing the herb produces histidine which neutralizes metals in test tubes, and finally concludes this must explain the herb's real-world survival ability.

Main Conclusion:

The herb's high histidine production must be what allows it to grow in metal-rich soils.

Logical Structure:

This is a causal argument that jumps from lab evidence (histidine neutralizes metals in test tubes) to a real-world explanation (histidine must be protecting the herb in nature). The key logical gap is assuming that what happens in a test tube also happens inside a living plant in soil.

Prethinking:

Question type:

Evaluate - We need to find what information would be most important to determine whether the conclusion is valid or not

Precision of Claims

The conclusion makes a causal claim that histidine production is THE KEY feature allowing survival in metal-rich soils, based on lab evidence that histidine neutralizes metals

Strategy

For evaluate questions, we need to think of assumptions the argument makes and create scenarios that would either strengthen or weaken the conclusion when we get more information. The argument jumps from 'histidine works in test tubes' to 'histidine must be why the herb survives in real soil.' We should look for gaps between lab conditions and real-world conditions, or alternative explanations for the herb's survival.

Answer Choices Explained

Whether the herb can thrive in soil that does not have high concentrations of the toxic metals

This doesn't help us evaluate whether histidine is the key feature for surviving in metal-rich soils. Even if the herb could grow in normal soil, this wouldn't tell us anything about what specifically allows it to survive in toxic conditions where other plants can't.

Whether others of the closely related group of plants also produce histidine in large quantities

This is crucial for evaluating the argument. The passage tells us this herb is part of a group of related plants that ALL thrive in metal-rich soils. If these other plants also produce high histidine levels, then histidine alone can't be what makes THIS particular herb special. If they don't produce histidine but still survive, this would directly contradict the conclusion that histidine is the key survival feature.

Whether the herb's high level of histidine production is associated with an unusually low level of production of some other amino acid

This might be interesting from a biochemical perspective, but it doesn't help us evaluate whether histidine is responsible for the herb's ability to survive in toxic soils. The trade-off between amino acids doesn't address the causal claim.

Whether growing the herb in soil with high concentrations of the metals will, over time, reduce their concentrations in the soil

This addresses what happens after the herb grows, but doesn't help us evaluate what allows the herb to survive initially in the toxic environment. The conclusion is about survival mechanism, not environmental cleanup effects.

Whether the concentration of histidine in the growing herb declines as the plant approaches maturity

This deals with histidine levels changing over the plant's lifecycle, but doesn't help evaluate whether histidine is what initially allows survival in metal-rich soils. The timing of histidine production doesn't test the core causal claim.

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