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 keys. It does not need another sentence for context. It gives a broad overview of what piano keys are made of. Why other lines are not chosen? Paragraph 2 starts with 'Spruce' and is specific, so it is not independent. Paragraph 3 starts with 'Traditionally' which is a transitional word, so it is not independent. Paragraph 4 starts with 'But' which is also a conjunction, so it is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about piano keys and their materials. It gives a conclusion about the current situation with ivory and plastics.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of piano keys. It uses the Logical Order rule, as it gives more specific information about the type of wood used.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it gives more details about the materials used for piano keys. This follows the Broader to Specific Strategy, as it moves from the general use of spruce to specific types of keys.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains the change in materials used for piano keys. It uses the Cause and Effect Relationship rule, as it shows why plastics are now used instead of ivory.