First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic, which is the mantle of the Earth. It gives a broad overview of what the mantle is and how it behaves. This matches the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "This leads," which is a pronoun that refers back to the previous sentence. Paragraph 3 starts with "The build-up," which also refers to earlier ideas. Paragraph 4 starts with "The movement," which again needs context from earlier sentences. All these sentences are not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the ideas about the movement of tectonic plates and their effects. It provides a final explanation of how the mantle behaves, which reflects the overarching theme.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains what happens because of the mantle's behavior. This shows a cause and effect relationship.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains the result of the tectonic plates moving. This shows a logical order of events.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains what happens after the earthquakes. This shows a cause and effect relationship.