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, which is the Black Death. It gives a broad overview of what the text will discuss. It does not use any pronouns or definite articles that would need prior context. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the criteria for being independent. For example, Paragraph 2 starts with 'It', which is a pronoun. Paragraph 3 starts with 'This', which also refers back to something. Paragraph 4 starts with 'A', which is an indefinite article but does not introduce the main topic. Paragraph 5 starts with 'Then', which is a conjunction. Paragraph 6 starts with 'Whereas', which is also a conjunction.

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 conclusion of the situation, highlighting the confusion of doctors. It uses the word 'Whereas', which indicates a contrast and signals the end of the explanation.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more detail about the cause of the Black Death. This follows the Broader to Specific Strategy, where the first paragraph gives a general idea and the second paragraph provides specific information.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains more about the flea mentioned in Paragraph 2. This follows the Logical Order rule, as it continues the explanation about the flea.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains the effect of the flea on humans. This follows the Cause and Effect Relationship rule, as it shows what happens when a human is bitten.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it describes how the disease spreads from one person to another. This follows the Chronological Order rule, as it shows the sequence of events.
• Pair 5 (Paragraph 5 → Paragraph 6): Paragraph 6 connects to Paragraph 5 because it shows the reaction of doctors to the spread of the disease. This follows the Contrast rule, as it contrasts the spread of the disease with the lack of knowledge from doctors.