A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first because it introduces the main topic of aviation progress in the 1920s and 1930s. It gives a broad overview of important events without needing prior context. It does not use any pronouns or definite articles that refer to previous information. Why other lines are not chosen? Paragraph 2 starts with "The war" which refers to a specific event, so it cannot stand alone. Paragraph 3 starts with "By the beginning of World War II" which also needs context. Paragraph 4 uses "the war" and is not independent because it assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last because it summarizes the innovations that occurred due to the war. It wraps up the discussion about aviation progress and reflects the overarching theme of advancements in aviation.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of aviation progress. It gives a specific example of a successful design after discussing general progress. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it shows the result of the successful designs like the DC-3. It explains how the aviation industry grew, which is a logical order of events.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it discusses the innovations that came during the war. This shows a cause and effect relationship, where the growth of airports and pilots leads to new innovations.