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Difficulty: MediumPart-to-Whole Rhetorical Function

During the African Humid Period, which ended approximately 5,000 years ago, the Sahara was a verdant landscape supporting diverse ecosystems and human settlements. Some climate models suggest that this transition to the modern hyper-arid desert occurred abruptly, triggered by a threshold-crossing feedback loop between vegetation cover and precipitation. However, recent geological evidence from dust deposition in marine sediments indicates a much more gradual, trans-regional aridification process that spanned several millennia. This discrepancy highlights the limitations of relying solely on simplified atmospheric simulations to reconstruct past ecological shifts.

Which choice best describes the function of the underlined sentence in the text as a whole?

  1. A
    It disputes the historical consensus that the Sahara was once a verdant landscape with diverse ecosystems.
  2. B
    It argues that vegetation feedback loops are the primary driver of all long-term global precipitation trends.
  3. It introduces a theoretical explanation of an ecological transition that the author subsequently contrasts with empirical findings.Answer
  4. D
    It states the main conclusion of the passage regarding the specific feedback loop that caused Sahara's desertification.

Answer

The choice stating that the underlined sentence introduces a theoretical explanation of an ecological transition that is subsequently contrasted with empirical findings is correct.
The underlined sentence describes a theoretical hypothesis generated by climate models to explain the transition of the Sahara from humid to arid. In the next sentence, the author introduces geological sediment evidence that indicates a gradual transition instead of an abrupt one. Thus, the underlined sentence serves to introduce a theoretical explanation of an ecological transition that is subsequently contrasted with empirical findings.

Step-by-Step Solution

1
Identify the claim made in the underlined sentence.
The underlined sentence states that some climate models suggest the transition to hyper-arid desert was abrupt and driven by a feedback loop.
Understanding the content of the specific part is the first step in determining its rhetorical function.
2
Analyze how the rest of the text responds to or builds on the underlined sentence.
The text immediately introduces contrasting evidence ('However, recent geological evidence...') indicating a gradual transition, and concludes that this shows the limits of the models.
The rhetorical function of a sentence depends on its relationship to the surrounding context.
3
Determine the objective logical relationship between the underlined sentence and the text as a whole.
The underlined sentence presents a theoretical model (abrupt feedback loop) that is subsequently countered by empirical evidence (gradual dust record). This matches the explanation that the sentence introduces a theoretical explanation that is later contrasted with empirical findings.
Matching the analyzed relationship to the correct description yields the correct answer.

Key Concept

Part-to-Whole Rhetorical Function
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Topics

Part-to-Whole Rhetorical Function

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