A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of Leonardo da Vinci and his drawings for a helicopter. It does not use any words that need prior context. It gives a broad overview of the subject. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "In fact," which is a connector. Paragraph 3 uses "The toy" which refers back to something not yet introduced. Paragraph 4 starts with "A string" which also needs context. Paragraph 5 starts with "When the string" which is a dependent clause.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it describes the final action of the toy. It wraps up the explanation of how the toy works.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more information about helicopters. The phrase "In fact" shows that it adds to the idea of helicopters.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it describes the toy mentioned in Paragraph 2. It gives specific details about the toy's design.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it continues to explain how the toy works. It gives more details about the parts of the toy.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it shows what happens when the string is pulled. It explains the action that follows the setup in Paragraph 4.