First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic. It talks about an important event that happened in 1956. This event sets the stage for the following information about air traffic control. It does not need another sentence for context. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "The resulting structure," which needs context from a previous sentence. Paragraph 3 uses "This resulted in," which also needs earlier information. Paragraph 4 starts with "Similar air traffic control procedures," which assumes knowledge of previous topics.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the topic. It shows that the information about air traffic control is not only for the US but also for other parts of the world. This gives a complete view of the subject.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it shows the result of the accident. The phrase "resulting structure" shows that the safety improvements came after the accident. This is a cause and effect relationship.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains what happened next. The phrase "This resulted in" shows that the establishment of the FAA came after the improvements in air traffic control. This is a logical order of events.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it expands on the topic of air traffic control. It shows that the procedures in the US are similar to those in other parts of the world. This is a broader to specific strategy.