A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic about lobsters and suffering. It does not need another sentence to understand it. It sets the scene for the discussion about pain in crustaceans. Why other lines are not chosen? Paragraph 2 starts with "Until recently" which needs prior context. Paragraph 3 starts with "Scientists" which is not general. Paragraph 4 starts with "And they found" which is a conjunction and needs more context.

Concluding sentence: Why this is the last line? This paragraph is the last line because it provides a conclusion to the discussion. It shows the findings of the research and wraps up the topic about pain in crustaceans.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 explains the belief about crustaceans' pain. It follows logically from the suffering mentioned in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 shows a change in thinking. It builds on the idea in Paragraph 2 about crustaceans and pain.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 gives an example of the research mentioned in Paragraph 3. It shows evidence of crustaceans' sensitivity to pain.