A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It uses the rule of Topic Sentence. It clearly states what gravitation is. 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 law of Newton', which needs prior context. Paragraph 3 starts with 'He then thought', which also needs context. Paragraph 4 uses 'This law', which refers back to something not yet introduced.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the text. It uses the Conclusive Words rule, as it gives the final name of the law discussed in the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of gravitation. It uses the Logical Order rule, as it explains who gave the law of gravitation.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains Newton's thoughts after he discovered the law. This follows the Cause and Effect Relationship rule, as it shows what Newton thought after learning about gravity.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it summarizes the law that Newton created. It uses the Broader to Specific Strategy, as it gives a general conclusion about the law of gravitation.