A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about krill and their food. It does not need another sentence for context. It gives a broad overview of what krill eat without going into details. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'While', which is a conjunction. Paragraph 3 uses 'Some scientists', which refers to a group and needs more context. Paragraph 4 mentions 'Part of the decline', which assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the reasons for the decline in krill populations. It wraps up the discussion by mentioning climate change and other factors.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of krill. The phrase 'While krill may last' shows a logical order of ideas about krill's survival.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it gives evidence about krill populations. The phrase 'Some scientists estimate' shows a cause and effect relationship, explaining why krill are in danger.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains the reasons for the decline in krill populations. The phrase 'Part of the decline' shows a logical order from the estimate to the causes.