First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of airline travel in the tourism industry. It is a general statement that does not need other sentences for context. It sets the stage for the discussion about air travel. 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 'This is achieved,' which needs more context. Paragraph 3 starts with 'However,' which is a conjunction. Paragraph 4 starts with 'Consequently,' which also needs prior information. Therefore, they cannot be the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about air travel and its impact. It shows the outcome of the previous points made about environmental concerns and the need for sustainability.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains how airline travel is made possible. The phrase 'This is achieved' refers back to the idea of airline travel being crucial, showing a logical flow.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it introduces a new concern about air travel. The word 'However' shows a contrast between the benefits of air travel and its environmental impact.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the result of the concern mentioned in Paragraph 3. The word 'Consequently' indicates that the pressure on the airline industry is a direct result of the environmental concerns.