A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic, which is the ancient city of Petra. It gives a broad overview of the city without going into details. It does not use any pronouns or definite articles that refer to previous information. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the criteria for being independent. - Paragraph 2 starts with 'Its', which is a pronoun. - Paragraph 3 starts with 'The', which is a definite article and assumes prior knowledge. - Paragraph 4 starts with 'Additionally', which is a conjunction.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about Petra. It adds a final thought about the mystery of how the Nabataeans built the city. This shows that the topic is closed.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of Petra. The phrase 'Its intricate water conduit system' refers back to Petra, showing a clear link between the two paragraphs. This follows the Noun Pronoun Rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it talks about the purpose of the water conduit system mentioned in Paragraph 2. This shows a logical order where one idea leads to another.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it adds more information about Petra. The word 'Additionally' shows that it is giving more details about the city, linking the two ideas.