A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of advertising. It does not need another sentence to understand it. It is a general statement about advertising. Why other lines are not chosen? Paragraph 2 starts with 'This' which is a pronoun and needs context. Paragraph 3 starts with 'The' which refers to something specific and needs prior knowledge. Paragraph 4 also starts with 'The' and is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about advertising. It gives a final thought on what marketers consider.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains what advertising does. It uses the connector 'This' to show the result of advertising.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it talks about a type of advertising. It shows a specific example of advertising that is less costly.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it adds more information about what marketers think. It shows that marketers look at other types of media.