A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the topic of early rails. It gives a broad overview of how these rails were used and their materials. It does not refer to any previous information, making it independent. Why other lines are not chosen? Paragraph 2 starts with 'These rails,' which refers back to the rails mentioned in Paragraph 1, so it is not independent. Paragraph 3 starts with 'However,' which is a conjunction and does not stand alone. Paragraph 4 starts with 'These were,' which also refers back to previous information. Paragraph 5 starts with 'An early proponent,' which is specific and does not introduce the topic.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about rail design. It gives specific examples of people who contributed to the design, which brings closure to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the discussion about the early rails. The phrase 'These rails' refers back to the rails mentioned in Paragraph 1, showing a clear link.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 with the word 'However,' which shows a contrast. It explains that even though the initial cost was low, the long-term costs were high.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 follows Paragraph 3 logically because it explains what happened next. The phrase 'These were superseded' refers to the rails discussed in Paragraph 3, showing a clear progression.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it introduces specific people related to the design of the rails. It continues the topic of rail design and shows how it evolved.