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Difficulty: MediumLogical Inferences

The analysis of fossilized leaves of the deciduous tree *Ginkgo biloba* from the Cretaceous-Paleogene transition reveals a sharp decrease in stomatal density—the number of pores per unit of leaf area. Because modern *Ginkgo* trees respond to elevated atmospheric carbon dioxide (CO2CO_2) concentrations by reducing their stomatal density to conserve water, some researchers conclude that global CO2CO_2 levels rose significantly during this period. However, skeptics suggest that this reduction was merely a localized adaptation to regional soil moisture levels in the specific basins where the fossils were deposited. This skeptical view is undermined, however, by the discovery of fossilized *Ginkgo* leaves from geographically isolated regions of the same era that also show uniform stomatal reduction, which suggests that

Which choice most logically completes the text?

  1. the stomatal changes were likely a response to a widespread atmospheric shift rather than localized soil factors.Answer
  2. B
    deciduous trees in the Cretaceous-Paleogene period were more resilient to changes in soil moisture than modern trees are.
  3. C
    elevated atmospheric carbon dioxide levels actually increased soil moisture in the geographically isolated regions.
  4. D
    the local soil moisture levels of the different depositional basins were identical during this transition period.

Answer

the stomatal changes were likely a response to a widespread atmospheric shift rather than localized soil factors.
The correct option correctly infers that because the stomatal density reduction is uniform across geographically separated basins, the cause is a global factor (such as atmospheric changes) rather than isolated local conditions.

Step-by-Step Solution

1
Identify the main argument and the objection raised in the passage.
The main argument is that decreasing stomatal density in Ginkgo leaves indicates rising global carbon dioxide. The objection is that this change was due to local soil moisture conditions in specific depositional basins.
To evaluate a logical inference, we must first map the opposing perspectives presented in the text.
2
Analyze the counterevidence presented against the objection.
Fossilized leaves from geographically isolated regions from the same era show the same uniform reduction in stomatal density.
This counterevidence directly tests whether the local environment can explain the phenomenon.
3
Draw the logical conclusion that follows from the counterevidence.
Because the reduction is uniform across different, geographically isolated regions, a local factor cannot be the primary cause. Instead, a global factor (like the atmosphere) must be responsible.
A uniform effect across isolated and distinct environments is logically explained by a common, widespread cause rather than multiple identical localized anomalies.

Key Concept

Identifying a logically necessary conclusion that resolves a conflict between localized and global environmental explanations.
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Topics

Logical Inferences

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