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Difficulty: MediumTransitions: Chronology and Sequence

During the Archean Eon, Earth's atmosphere was largely composed of volcanic gases such as methane, ammonia, and carbon dioxide, with virtually no free oxygen. Around 2.4 billion years ago, photosynthetic cyanobacteria emerged and began producing oxygen as a byproduct of photosynthesis. ________, the oceans became saturated with oxygen, and the excess gas accumulated in the atmosphere, triggering the Great Oxidation Event.

Which choice completes the text with the most logical transition?

  1. Gradually,Answer
  2. B
    In contrast,
  3. C
    In particular,
  4. D
    Previously,

Answer

Gradually,
The passage describes a chronological progression over time. The emergence of cyanobacteria led to the slow accumulation of oxygen, which eventually saturated the oceans and atmosphere. 'Gradually' is the correct transition because it accurately reflects this progressive, step-by-step chronological development.

Step-by-Step Solution

1
Analyze the relationship between the sentences before and after the blank.
The sentence before the blank describes the emergence of cyanobacteria and their production of oxygen. The sentence after the blank describes the saturation of the oceans and the accumulation of oxygen in the atmosphere.
Identifying the logical relationship between ideas is necessary to select the correct transition word.
2
Determine the chronological relationship of the events.
The accumulation of oxygen is a subsequent event that occurred over a long period following the emergence of cyanobacteria.
This establishes that a chronological or sequential transition indicating progression over time is needed.
3
Evaluate the choices to find the transition that correctly indicates a progressive, sequential timeline.
The word 'Gradually' correctly shows that the process developed incrementally over time. Other options misrepresent the relationship by indicating contrast, elaboration, or a reversed chronological order.
Eliminating incorrect transition relationships leads to the correct choice.

Key Concept

Transitions: Chronology and Sequence
Estimated Time:1m 0s
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Topics

Transitions: Chronology and Sequence

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