A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic, which is the Black Death. It gives a broad overview of the disease without going into details. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'It was probably started...' which uses a pronoun and needs context. Paragraph 3 starts with 'This flea...' which also uses a pronoun. Paragraph 4 starts with 'A bite from one of these fleas...' which is specific and does not introduce the topic. Paragraph 5 starts with 'Then, as the disease spread...' which indicates it is not the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the spread of the disease. It shows the final effect of the Black Death, which is the main topic of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains how the disease started. This follows the Logical Order rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives more detail about the flea mentioned in Paragraph 2. This follows the Broader to Specific Strategy.
• 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.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it shows how the disease spread from one person to another. This follows the Logical Order rule.