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, which is the layout of US cities. It gives a clear overview of what to expect in the following paragraphs. It does not refer to any previous information, making it independent. 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 'In cities elsewhere,' which refers to other cities and does not stand alone. Paragraph 3 starts with 'This is not simply a case,' which refers back to something not yet mentioned. Paragraph 4 starts with 'The ancient Romans,' which assumes prior knowledge of Romans.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about city layouts. It gives a clear example of how a grid pattern was created by the Romans, which ties back to the main topic of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it shows a contrast. Paragraph 1 talks about the grid pattern in US cities, while Paragraph 2 describes the meandering pattern in European cities. This is a clear use of the Contrast rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it continues the discussion about city layouts. It explains that the differences in patterns are not just about age. This shows a Logical Order as it builds on the idea presented in Paragraph 2.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it provides an example of a grid pattern. It explains how the Romans created their streets, which supports the idea in Paragraph 3. This is an example of the Topic Example rule.