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 Nile River. It gives a broad overview of the river and its importance. It does not need any other sentence for context. Why other lines are not chosen? Paragraph 2 starts with "Its principal tributaries..." which uses a pronoun and needs context. Paragraph 3 starts with "The former..." which also uses a pronoun and refers back to something not yet mentioned. Paragraph 4 starts with "Then these two tributaries..." which uses a transitional word and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the information about the Nile and its tributaries. It shows the final part of the river's journey, which concludes the discussion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 gives specific information about the Nile River. It follows logically after the introduction of the Nile in Paragraph 1. This is a Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 explains more about the two tributaries mentioned in Paragraph 2. It uses the Noun Pronoun Rule, where "the former" and "the latter" refer back to the White Nile and the Blue Nile.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 describes what happens after the explanation of the tributaries in Paragraph 3. It follows a logical order, showing the next step in the river's journey.