A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of the text. It gives a broad overview of how mass transportation changed American cities. It does not refer to any previous information, so it stands alone. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'It', which is a pronoun that refers back to something. Paragraph 3 starts with 'In 1850', which gives a specific time and does not introduce the main topic. Paragraph 4 starts with 'Now', which shows a change and does not stand alone. Paragraph 5 starts with 'The new accessibility', which uses 'the' and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the main ideas of the text. It shows the result of the changes discussed in the previous paragraphs. This follows the Summary Style rule, as it brings together the earlier points about mass transportation and urban development.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains the three ways mentioned in the first paragraph. This follows the Broader to Specific Strategy. The first paragraph gives a general idea, and the second paragraph gives specific details about how mass transportation changed cities.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives an example of the physical expansion mentioned in the second paragraph. This follows the Topic Example Strategy. The second paragraph talks about expansion, and the third paragraph shows how this happened in Boston.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains what happened after the expansion of Boston. This follows the Logical Order Strategy. The third paragraph gives a historical fact, and the fourth paragraph shows the result of that fact.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it explains why people could live far from the city center. This follows the Cause and Effect Relationship Strategy. The fourth paragraph shows the situation, and the fifth paragraph explains the reason behind it.