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 gives a broad overview of the connection between art and nature in singing. It does not use any specific details or references to other ideas. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'A perfect voice,' which is specific and needs context. Paragraph 3 uses 'There is no evidence,' which also needs prior information. Paragraph 4 starts with 'Yet if the accepted idea,' which shows a contrast and needs earlier context. Paragraph 5 starts with 'Yet if the accepted idea,' which also needs prior context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the ideas presented in the text. It gives a conclusion about the relationship between spontaneity and effort in singing.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 gives an example of what a perfect voice is like. It follows the general idea of art and nature in singing from Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 explains how the tones of a perfect voice are natural and free. It continues the idea from Paragraph 2 about the qualities of a perfect voice.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 explains that the free tones mentioned in Paragraph 3 do not show effort. It continues the thought about the nature of a perfect singer.
• Pair 4 (Paragraph 4 → Paragraph 5): This pair connects because Paragraph 5 explains that the appearance of spontaneity in singing comes from hard work and rules. It shows a cause and effect relationship from the previous paragraph.