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Difficulty: HardEstablishing Introductions and Thesis Statements

The following is an excerpt from an essay about North African paleoclimate.

[1]
__________________________________________________ Archaeological excavations in the region have uncovered remains of hippopotamuses, crocodiles, and aquatic plants, alongside ancient fishing tools and pastoral rock art depicting long-horned cattle. These discoveries indicate that between 11,00011,000 and 5,0005,000 years ago, a massive lake system—including a "Mega-Chad" that stretched across 350,000350,000 square kilometers—supported diverse ecosystems. Furthermore, fossilized pollen reveals that Mediterranean vegetation, such as oaks and wild olives, once flourished where sand dunes now drift.

[2]
This dramatic ecological shift was primarily driven by gradual variations in the Earth's orbit, known as Milankovitch cycles. These orbital changes altered the distribution of solar energy, intensifying the West African monsoon and drawing moisture-laden winds deep into the continent's interior. When the orbit shifted back, the monsoon weakened, triggering a rapid feedback loop between declining vegetation and rising surface reflectivity that ultimately reclaimed the landscape for the desert.

Which choice provides the most effective introduction to the passage by establishing its central focus?

  1. A
    Although the Sahara is currently the largest hot desert in the world; scientists have discovered that its climate was once vastly different.
  2. During a period known as the African Humid Period, the now-arid Sahara Desert was a fertile landscape of lakes, rivers, and grasslands.Answer
  3. C
    Because the Sahara Desert contains vast quantities of fossilized pollen and ancient fishing tools, which indicate that the region was not always dry.
  4. D
    Earth's orbit, which fluctuates over thousands of years, is responsible for major climate cycles, and these shifts have a profound impact on global weather patterns.

Answer

During a period known as the African Humid Period, the now-arid Sahara Desert was a fertile landscape of lakes, rivers, and grasslands.
The correct option effectively introduces the passage by establishing its central focus on the past wet, fertile state of the Sahara Desert. This provides a direct transition into the first paragraph's discussion of archaeological evidence (hippopotamuses, lakes, Mediterranean pollen) and the second paragraph's explanation of orbital changes.

Step-by-Step Solution

1
Analyze the passage to determine its main subject.
The passage describes archaeological and ecological evidence (lakes, pollen, animal remains) proving the Sahara was once wet and green, and explains the orbital mechanics behind this transition.
An effective introduction must establish the specific context and focus of the entire passage.
2
Evaluate the rhetorical focus of the options.
The option referring to the African Humid Period and the fertile past of the Sahara introduces all the key elements of both paragraphs.
An introduction must align with the tone and details that follow.
3
Check the options for grammatical correctness.
The correct option is grammatically sound, whereas the other options contain errors such as fragments or incorrect semicolon usage.
The correct answer in ACT English must be both rhetorically appropriate and grammatically correct.

Key Concept

Establishing Introductions and Thesis Statements
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Topics

Establishing Introductions and Thesis Statements

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