A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of piano construction. It gives a broad overview of how keys were made in the past. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with 'Today' which makes it dependent on prior context. Paragraph 3 starts with 'Also,' which shows it is not independent. Paragraph 4 starts with 'Also,' which also makes it dependent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about materials used for piano keys. It gives a final point about the disadvantages of ivory compared to plastic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because it shows a logical order of time. The first paragraph talks about the past, and the second paragraph talks about the present. This is a clear chronological order.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because it continues the topic of piano keys. The second paragraph talks about the materials used today, and the third paragraph explains the traditional materials used before. This shows a broader to specific strategy.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because the fourth paragraph gives more detail about ivory. It explains a problem with ivory compared to plastic. This is a cause and effect relationship.