A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of hair and its parts. It gives a broad overview of what hair is made of, which is important for understanding the rest of the text. It does not use any words that need prior context. Why other lines are not chosen? Paragraph 2 starts with "The matrix" which uses a definite article and refers back to the matrix mentioned in Paragraph 1, so it is not independent. Paragraph 3 starts with "The combination" which also uses a definite article and refers back to the components mentioned before. Paragraph 4 starts with "And as hair" which is a conjunction and does not stand alone. Paragraph 5 starts with "This structural change" which uses a pronoun that refers back to the change mentioned in previous sentences.

Concluding sentence: Why this is the last line? This paragraph is the last line because it summarizes the main idea of how hair can withstand stress. It wraps up the discussion by showing the result of the structural changes in hair.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it talks about the matrix, one of the two parts mentioned in Paragraph 1. This shows a logical order where the details about the matrix follow the introduction of the parts of hair.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains the importance of the matrix and cortex together. It shows how these two parts work together to give hair strength, which is a logical continuation of the discussion.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it describes what happens when hair is under stress. It logically follows the explanation of how the components help hair withstand stress.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it explains the result of the structural change mentioned in Paragraph 4. It shows the effect of the stretching on hair, which is a clear cause and effect relationship.