A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of skin classification. It does not need another sentence for context. It is a general statement that sets up the discussion about skin types. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the criteria for being independent. For example, Paragraph 2 starts with "For example," which is a conjunction. Paragraph 3 starts with "It is used," which uses a pronoun that refers back to a previous idea. Paragraph 4 starts with "Based on his method," which also refers back to something not yet introduced.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the main points about skin types. It gives a clear conclusion about the classification method discussed in the previous paragraphs.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives an example of the criteria mentioned. The word "For example" shows this connection.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains how Fitzpatrick's classification is used. The word "It" refers back to the classification mentioned in Paragraph 2.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives specific types of skin based on the classification. The phrase "Based on his method" shows that it is following the discussion about classification.