A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main idea about how the Earth was formed. It does not need any other sentence for context. It is a general statement that sets the stage for the following information. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'As this boiling mass...' which is a conjunction. Paragraph 3 starts with 'In either case...' which also needs context. Paragraph 4 starts with 'Some believe...' which is not general enough to be the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it describes the final process of the Earth's formation. It wraps up the ideas about how the Earth formed and what happened to its elements. It gives a clear ending to the explanation.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of how the Earth was formed. It uses the phrase 'Some believe' which links back to the belief mentioned in Paragraph 1. This shows a logical order of thought.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it explains what happened after the Earth condensed. The phrase 'In either case' shows that it relates to the previous belief. This is a logical order of ideas.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it describes what happened to the molten mass. The phrase 'As this boiling mass' shows a continuation of the story. This is a logical order.