A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of sustainable travel. It sets the stage for what the reader will learn about. It uses a broad overview to explain that research is important for sustainable travel. Why other lines are not chosen? The other paragraphs do not qualify as the first sentence. For example, Paragraph 2 starts with 'After selecting,' which means it needs prior context. Paragraph 3 starts with 'Packing light,' which is specific and does not introduce the topic. Paragraph 4 starts with 'By choosing local experiences,' which also needs context from earlier sentences.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the ideas about sustainable travel. It shows how tourists can make a positive impact, which is the main theme of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it follows the logical order of steps in sustainable travel. After understanding the destination, the next step is to look for eco-friendly options.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it continues the topic of eco-friendly travel. Packing light is another way to be eco-friendly.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it adds another way to support sustainable travel. Choosing local experiences is also part of being eco-friendly.