A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of coral reefs. It gives a broad overview of coral reefs and their importance in marine life. It does not use any pronouns or definite articles that refer to previous information. Why other lines are not chosen? Paragraph 2 starts with 'But' which is a conjunction, so it cannot be independent. Paragraph 3 starts with 'What is more,' which is also a connector, making it dependent. Paragraph 4 starts with 'In combination with,' which is a transitional phrase, so it is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last line because it summarizes the main ideas and presents a conclusion about the threats to coral reefs. It uses the phrase 'we start to arrive at some quite frightening scenarios,' which indicates a wrap-up of the discussion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 using the 'Connector Strategy' because it starts with 'But,' which shows a contrast to the previous idea about marine life. It adds more information about the importance of coral reefs.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 using the 'Connector Strategy' with 'What is more,' which adds more information about the livelihoods mentioned in Paragraph 2. It shows a logical order of increasing importance.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 using the 'Cause and Effect Relationship' rule. The expected doubling of the number of people relying on reefs leads to the threat of climate change, showing a logical progression.