A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic, which is the Sahara Desert. It gives a broad overview of what the text is about. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with "It" which is a pronoun, so it cannot be independent. Paragraph 3 starts with "Despite" which is a conjunction, so it cannot be independent. Paragraph 4 starts with "Besides" which is also a conjunction, so it cannot be independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the Sahara Desert. It gives a final point about its importance, which is a summary style of concluding.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more details about the Sahara Desert. This follows the Broader to Specific Strategy, where the first sentence introduces the desert and the second sentence describes its features.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it talks about life in the desert despite its harsh conditions. This shows a Cause and Effect Relationship, where the harsh climate leads to the need for special adaptations in plants and animals.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it adds another important point about the desert. This follows the Logical Order strategy, where the discussion about the desert's life leads to its importance in trade.