A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first because it introduces the main topic of the text. It shows the importance of physical activity for mental and emotional health. This is a broad overview and sets the stage for the following sentences. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'Physical activity releases endorphins,' which needs the context from the first paragraph. Paragraph 3 mentions 'It has been shown,' which refers back to something not yet introduced. Paragraph 4 starts with 'Additionally,' which shows it is adding to a previous idea.

Concluding sentence: Why this is the last line? This paragraph is the last because it wraps up the discussion about exercise. It summarizes the importance of exercise for both mental and physical health. The word 'Additionally' shows that it is adding to the previous points, making it a good conclusion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains how physical activity helps mental health. The phrase 'Physical activity releases endorphins' shows a cause and effect relationship. This means that the recognition of physical activity leads to understanding its benefits.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives evidence of the benefits of physical activity. The phrase 'It has been shown' refers back to the idea of endorphins helping mood, showing a logical order from cause to effect.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it adds more information about exercise. The word 'Additionally' shows that this paragraph is giving more reasons why exercise is important, linking the ideas together.