First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the topic of hot-air balloons. It gives a general statement about their history without needing other sentences for 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 "Yet" which is a conjunction, so it cannot be independent. Paragraph 3 uses "The" which assumes prior knowledge, making it not independent. Paragraph 4 also uses "The" and is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about balloon technology. It gives a final point about the success of the Statoscope, which relates back to the earlier paragraphs. This is a Summary Style strategy.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it uses the word "Yet". This shows a contrast between the old hot-air balloons and modern scientific balloons. This is a Contrast strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 logically because it talks about the development of modern balloons. This shows a flow from the idea of modern balloons to who is developing them. This is a Logical Order strategy.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it talks about the success of the Statoscope, which is related to the development mentioned in Paragraph 3. This shows a continuation of the topic. This is a Topical Order strategy.