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 talks about travel agencies and tour operators, which are important in tourism. This is a general statement that does not need other sentences for context. It does not start with a conjunction or use pronouns. 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. Paragraph 3 starts with 'Initially', which is a transitional word. Paragraph 4 starts with 'As a result', which is also a transitional phrase.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the main idea of the text. It shows the outcome of the changes in travel services. It uses the phrase 'As a result', which indicates that it is concluding the discussion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains what travel agencies and tour operators do. This is a logical order where the general statement about travel agencies is followed by specific details about their services.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it gives more details about the services mentioned. It shows how these services have changed over time, which is a logical flow from what they do to how they have evolved.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the result of the changes mentioned in Paragraph 3. It explains how the shift to online services has changed the way tourists plan their trips.