A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first because it introduces the main topic of the text. It gives a broad overview of how the Earth was formed. It does not refer to any previous information, so it stands alone. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with "Initially," which is a transitional word. Paragraph 3 starts with "Then," which also shows a sequence. Paragraph 4 uses "It formed," which refers back to something not yet introduced. All these reasons make them not independent.

Concluding sentence: Why this is the last line? This paragraph is the last because it wraps up the ideas about the Earth's formation. It shows the final result of the processes described in the previous paragraphs. It summarizes the main theme of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the idea of the Earth's formation. It uses the word "Initially" to show the next step in the process of cooling.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it describes what happened after the Earth cooled. The word "Then" shows the order of events.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains what happened after the rain fell. The phrase "It formed seas" shows the result of the rain.