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 a biography. It gives a clear overview 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 as the first sentence. For example, Paragraph 2 starts with 'It', which is a pronoun and needs context. Paragraph 3 starts with 'Biographers', which is specific and not general. Paragraph 4 starts with 'The research', which uses 'the' and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about research for biographies. It gives specific examples of how research can be done, which concludes the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives more details about what a biography includes. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains what biographers must do to write a biography. This follows the Logical Order rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it describes the type of research biographers need to do. This follows the Cause and Effect Relationship rule.