First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about trees and their role in the ecosystem. It is a general statement that does not need other sentences for context. It sets up the main idea without going into details. 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 'They', which is a pronoun and needs context. Paragraph 3 starts with 'Moreover', which is a conjunction. Paragraph 4 starts with 'However', which is also a conjunction. These words show that they are not independent sentences.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it concludes the discussion by presenting a significant problem. It wraps up the ideas about trees and their importance by showing the threat they face.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of trees. The phrase 'They also' shows that it is adding more information about trees. This is a logical order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it adds another important point about trees. The word 'Moreover' shows that it is giving more information about the benefits of trees.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it introduces a contrasting idea. The word 'However' shows that it is talking about a problem related to trees, which is deforestation.