A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it is a clear topic sentence. It introduces the main idea about air travel changing over time. It does not need any other sentence for context. Why other lines are not chosen? Paragraph 2 starts with "Initially" which is a transitional word, so it cannot be independent. Paragraph 3 is not chosen because it refers to a change but does not introduce the main topic. Paragraph 4 starts with "However" which is also a conjunction, making it not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about air travel. It shows the final thoughts on the changes and their effects. It uses the word "However" to signal a conclusion about the concerns that come with the changes.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more detail about how air travel has changed. It explains that air travel was once a luxury but is now common. This shows a logical order from a general statement to a specific example.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains why air travel has changed. It gives reasons for the change mentioned in Paragraph 2. This follows a logical order from a specific example to the reasons behind it.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it talks about the effects of the changes mentioned. It shows a cause and effect relationship, where the changes in air travel lead to new concerns.