First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of air cargo transportation. It is a general statement that does not need another sentence for context. It does not start with a conjunction or use pronouns. It also does not use the definite article 'the' for defined subjects. Why other lines are not chosen? Paragraph 2 starts with 'They', which is a pronoun and needs context. Paragraph 3 starts with 'This', which also needs context. Paragraph 4 starts with 'Therefore', which is a conjunction and shows a conclusion, not an introduction.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the main ideas. It uses the word 'Therefore', which signals a conclusion. It brings together the importance of air cargo and its role in the global economy.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of air cargo. The phrase 'They provide' refers back to 'Airplanes', showing a clear link.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains why fast shipping is important. The word 'This' refers back to the fast shipping mentioned in Paragraph 2.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it summarizes the importance of air cargo. The word 'Therefore' shows that it is a conclusion based on the previous points.