A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of river deltas. It does not refer to any previous information. It is a general statement about river deltas. Why other lines are not chosen? Paragraph 2 starts with "The river" which makes it dependent on prior context. Paragraph 3 starts with "The changes" which also needs context. Paragraph 4 starts with "An island" which is specific and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it provides a specific example of the changes in the delta. It wraps up the discussion by showing a clear result of the changes mentioned before.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains how river deltas change. It uses the connector "the river" to show the process of building up and wearing down.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it talks about changes in a specific delta. The word "however" shows a contrast between general changes and the specific changes in China.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives an example of a specific change in the delta. The mention of an island shows a specific result of the changes discussed.