A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It gives a clear overview of what the text is about, which is the Black Death. It does not refer to any previous information, making it independent. This follows the rule of Topic Sentence. Why other lines are not choosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'As the disease spread,' which needs context. Paragraph 3 uses 'They did not understand,' which refers back to something not yet mentioned. Paragraph 4 starts with 'About one fifth,' which is specific and needs a topic. Paragraph 5 starts with 'Eventually,' which shows a sequence and needs earlier information.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the Black Death. It shows the final outcome of the disease over time. The phrase 'but it came back' indicates a conclusion to the earlier points made in the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of the disease. The phrase 'As the disease spread' shows a logical order from the introduction of the disease to how it affected people.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 links to Paragraph 2 because it explains more about the doctors' confusion. The phrase 'They did not understand' continues the idea of the doctors' lack of knowledge.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives a specific fact about the impact of the disease. The statement about the death toll follows the discussion of the doctors' understanding.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 follows Paragraph 4 because it shows the long-term effects of the disease. The word 'Eventually' indicates a chronological order, moving from the death toll to the ongoing outbreaks.