A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of polar bears. It gives a broad overview of what polar bears are and their importance. It does not use any words that need prior context, making it a clear starting point. Why other lines are not chosen? Paragraph 2 starts with "But" which is a conjunction, so it cannot be first. Paragraph 3 uses "Its" which is a pronoun and needs context. Paragraph 4 also starts with "But" and has a definite article "the" which assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion about polar bears. It gives a conclusion about their status as a threatened species due to climate change. This shows the overall theme of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more information about polar bears. It uses the Noun Pronoun Rule, where "Its" refers back to polar bears. This shows a clear link between the two sentences.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains why the name "sea bear" is suitable. This follows the Broader to Specific Strategy, where the second sentence gives more detail about the first.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it shows a problem that affects polar bears. It uses a Cause and Effect Relationship, where the previous information about polar bears leads to the issue of their habitat loss.