A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It clearly states what the text is about: sun energy. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs do not qualify as the first sentence. For example, Paragraph 2 starts with 'It', which is a pronoun. This means it cannot stand alone without context. Paragraph 3 starts with 'Sun energy', but it is not a general statement. It is more specific. Paragraph 4 starts with 'Therefore', which is a conjunction. This means it connects to something before it.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the main idea of the text. It shows the overall importance of sun energy. This follows the Summary Style rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains what sun energy does. This follows the Broader to Specific Strategy. The first sentence gives a general idea, and the second sentence gives more detail about that idea.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it adds more information about the effects of sun energy. This follows the Logical Order rule. The second sentence talks about climate, and the third sentence talks about plants, which are both affected by sun energy.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the importance of sun energy. This follows the Cause and Effect Relationship rule. The third sentence explains how sun energy helps plants, and the fourth sentence explains why that is important for life.