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 supersonic transport program. It gives a broad overview of the program and why it ended. It does not refer to any previous information, so it stands alone. Why other lines are not chosen? Paragraph 2 starts with "Declining ticket sales..." which is not independent because it uses a definite article "the" and refers to the program without introducing it. Paragraph 3 starts with "A fatal air crash..." which is also not independent because it uses the definite article "the". Paragraph 4 starts with "It returned to service..." which uses the pronoun "it" and needs context from earlier sentences.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the story of the supersonic transport program. It shows the final outcome after the events discussed in the previous paragraphs.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains why the program ended. It uses the logical order of ideas, showing the cause of the program's demise after stating the reason for its end.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it adds another reason for the program's end. It follows the logical order of events, showing that the crash was another factor affecting the program.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it continues the story of the plane after the crash. It uses chronological order, showing what happened next after the grounding.