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 tells us what the text is about: the Black Death. This follows the rule of Topic Sentence. It does not need any other 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 'As the disease spread,' which is a conjunction. Paragraph 3 uses 'They' which is a pronoun that refers back to something. Paragraph 4 starts with 'About one fifth,' which is not general. Paragraph 5 talks about what happened later, so it does not introduce the topic.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the story of the Black Death. It shows what happened after the main events. This follows the Summary Style rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of the disease. It explains how the disease spread. This follows the Logical Order rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it talks about what the doctors did not understand. This follows the Noun Pronoun Rule, as 'They' refers back to 'Doctors'.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it gives a result of the disease. It shows the effect of not understanding the disease. This follows the Cause and Effect Relationship rule.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it talks about what happened after the deaths. It shows the continuation of the story. This follows the Logical Order rule.