A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of World Mental Health Day. It gives a clear starting point with a specific date. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'It' which is a pronoun and needs context. Paragraph 3 starts with 'For a brief definition' which is not a general statement. Paragraph 4 starts with 'This year's theme' which also needs context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about World Mental Health Day. It gives a specific theme that relates to the overall topic. This follows the rule of Summary Style.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the idea of World Mental Health Day. It uses the Logical Order rule, as it shows that the event has been celebrated since the date mentioned.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains more about World Mental Health Day. This follows the Broader to Specific Strategy, as it gives a general idea and then details the purpose of the day.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives a specific theme related to the purpose mentioned in Paragraph 3. This follows the Topic Example rule, as it provides an example of the theme after explaining the purpose.