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Difficulty: MediumContextual Vocabulary and Pronoun Reference

In the early 1970s, marine geologists investigating oceanic spreading centers discovered hydrothermal vents, ecosystems operating entirely independent of solar energy. Primary production in these abyssal zones relies not on photosynthesis, but on chemosynthesis performed by sulfur-oxidizing endosymbiotic bacteria. These microbes inhabit the specialized trophosome tissue of giant tube worms (*Riftia pachyptila*), which lack a mouth, gut, or digestive tract. To sustain these organisms, the worms utilize specialized hemoglobin that simultaneously binds hydrogen sulfide and oxygen from seawater, transporting both toxic gases through their circulatory fluid without precipitating sulfur or poisoning cellular respiration. Consequently, the physiological survival of the host species depends strictly on facilitating the metabolic requirements of its microscopic symbionts. Although early researchers hypothesized that such metabolic adaptations represented primitive evolutionary precursors to photosynthetic pathways, contemporary phylogenomic analysis suggests they are actually highly derived specializations. Ultimately, the intricate exchange between the worm and its internal inhabitants demonstrates how extreme environmental selective pressures can drive radical morphological simplification in macroorganisms alongside complex biochemical co-evolution.

Based on the passage, the pronoun "they" in the penultimate sentence refers to which of the following?

  1. the metabolic adaptations of the hydrothermal vent organismsAnswer
  2. B
    the early researchers investigating hydrothermal vent ecosystems
  3. C
    the photosynthetic pathways operating in solar-dependent environments
  4. D
    the primitive evolutionary precursors described by early hypotheses
  5. E
    the contemporary phylogenomic analyses evaluating evolutionary history

Answer

The pronoun 'they' refers to the metabolic adaptations of the hydrothermal vent organisms.
The correct option accurately identifies 'metabolic adaptations' as the antecedent of the pronoun 'they'. Grammatically, 'they' is a plural pronoun that refers back to the plural subject 'such metabolic adaptations' introduced in the preceding dependent clause. Logically, the sentence contrasts what early researchers thought those adaptations were (primitive precursors) with what contemporary analysis suggests those same adaptations are (highly derived specializations).

Step-by-Step Solution

1
Locate the targeted pronoun in the penultimate sentence.
The target sentence reads: 'Although early researchers hypothesized that such metabolic adaptations represented primitive evolutionary precursors to photosynthetic pathways, contemporary phylogenomic analysis suggests they are actually highly derived specializations.'
Identifying the target sentence provides the immediate grammatical context.
2
Analyze the clause structure and agreement constraints.
The pronoun 'they' is third-person plural and functions as the subject of the that-clause following 'suggests'. It requires a plural noun antecedent.
Grammatical number and syntactical position restrict the possible referents.
3
Trace back to the logical plural noun phrase evaluated in the comparison.
The early researchers hypothesized that 'such metabolic adaptations' were precursors; the contrastive clause states that modern analysis suggests 'they' (the metabolic adaptations) are actually highly derived specializations.
The contrast between early hypotheses and contemporary findings centers on the evolutionary nature of the metabolic adaptations.

Key Concept

Identifying pronoun antecedents using syntactical alignment and logical contrast in dense complex sentences.
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Topics

Contextual Vocabulary and Pronoun Reference

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