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 Great Wall of China. It gives a broad overview of when the wall was built, which is important for understanding the history. It does not use any pronouns or definite articles that would need prior context. Why other lines are not chosen? Paragraph 2 starts with 'Its construction,' which uses a pronoun that refers back to the wall, so it cannot be independent. Paragraph 3 starts with 'Over centuries,' which is not a general statement and does not stand alone. Paragraph 4 starts with 'Despite its strength,' which also uses a pronoun and does not provide a general idea.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the Great Wall. It summarizes the challenges faced by the wall, which reflects the overall theme of its construction and purpose.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains the purpose of the wall's construction. This follows the 'Broader to Specific Strategy' where the first paragraph gives a general idea of the wall's building, and the second paragraph provides specific information about why it was built.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it shows how the wall was developed over time after its initial construction. This follows the 'Logical Order' rule, as it continues the narrative of the wall's importance and development.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it discusses the challenges the wall faced despite its significance. This follows the 'Contrast' rule, as it presents a contrasting idea about the wall's effectiveness.