First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the topic of the Earth's formation. It gives a broad overview of the time when the Earth was formed. It does not use any pronouns or definite articles that would need prior context. Why other lines are not chosen? Paragraph 2 starts with 'But', which is a conjunction. Paragraph 3 also starts with 'There', which is a pronoun. Paragraph 4 uses 'the' to refer to 'layers', which assumes prior knowledge. Paragraph 5 starts with 'In fact', which is a transitional phrase. All these reasons make them not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the Earth's layers. It summarizes the earlier points and provides closure to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 gives specific details about the Earth after its formation. It follows the broader statement in Paragraph 1 with specific facts, showing a General→Specific progression.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 introduces contrasting information. It uses 'But' to show a change in focus from the lack of life to the violent events that occurred, indicating a contrast.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 explains the result of the violent events mentioned in Paragraph 3. It provides a cause-and-effect relationship, showing how the early conditions of the Earth led to the separation of elements.
• Pair 4 (Paragraph 4 → Paragraph 5): This pair connects because Paragraph 5 summarizes the process described in Paragraph 4. It uses the phrase 'This process' to refer back to the separation of elements, showing a logical order from explanation to conclusion.