A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of dry skin and its causes. It does not refer to any previous information and gives a broad overview of the topic. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'These include,' which needs context from a previous sentence. Paragraph 3 starts with 'However,' which shows a contrast and needs earlier information. Paragraph 4 starts with 'These people,' which refers back to a group mentioned before.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about dry skin. It gives a final detail about the people who may have dry skin, which brings closure to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives specific examples of the external factors mentioned in Paragraph 1. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it introduces a contrast. It shows that while many people have temporary dry skin, some may have it more often. This follows the Contrast rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it describes the type of people mentioned in Paragraph 3. It explains that these people have sensitive skin. This follows the Noun Pronoun Rule.