A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of piano keys. It gives a broad overview of what materials are used for piano keys without going into details. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'Spruce' which is a specific term and does not introduce the topic. Paragraph 3 starts with 'Traditionally' which shows it is not independent. Paragraph 4 starts with 'Also' which is a conjunction and needs more context.

Concluding sentence: Why this is the last line? This paragraph is the last line 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. This follows the Conclusive Words rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of materials used for piano keys. This follows the Logical Order rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it gives more information about the materials used for piano keys. This follows the Broader to Specific Strategy.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it gives a reason why plastic is used instead of ivory. This follows the Cause and Effect Relationship rule.