A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about how the Earth was formed. It does not need any other sentence for context. It is a general statement that sets up the theme of the text. Why other lines are not chosen? Paragraph 2 starts with 'Some believe' which is not independent because it refers back to the previous idea. Paragraph 3 starts with 'In either case' which is also not independent because it needs context from earlier sentences. Paragraph 4 starts with 'As this boiling mass' which is not independent because it refers to the mass mentioned before.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the explanation of the Earth's formation. It describes the final process of how the heaviest elements settled in the centre, which concludes the overall theme.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the idea of how the Earth was formed. It uses the phrase 'Some believe' which shows a link to the previous thought.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains what happened after the Earth formed. The phrase 'In either case' shows that it relates to the previous ideas about the formation.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it describes what happened to the molten mass. The phrase 'As this boiling mass' shows that it continues the story from the previous paragraph.