A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of rivers and their importance. It gives a broad overview of what the text will discuss. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'They', which is a pronoun and needs context. Paragraph 3 starts with 'As', which is a conjunction. Paragraph 4 starts with 'Therefore', which is also a conjunction. These words show that they depend on previous sentences.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the main ideas. It shows the importance of rivers in agriculture. This follows the Conclusive Words rule, as it uses 'therefore' to summarize the points made earlier.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it gives more details about rivers. This follows the Broader to Specific Strategy. The first sentence talks about the role of rivers, and the second sentence explains where rivers come from.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it continues the idea of rivers. It explains what happens as rivers flow. This follows the Logical Order rule, as it shows the process of rivers affecting the land.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the result of what rivers do. It explains the importance of rivers in agriculture. This follows the Cause and Effect Relationship rule, as it shows how rivers help create fertile land.