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 credit system. It does not refer to any previous information and gives a broad overview of what the credit system does. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with "In this system," which refers back to the credit system, so it cannot be first. Paragraph 3 starts with "While credit," which also refers back to the topic. Paragraph 4 starts with "Therefore," which shows it is concluding something, so it cannot be first.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the credit system. It uses conclusive words like "therefore" to show it is summarizing the main idea.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains more about the credit system. It uses the Noun Pronoun Rule, as "this system" refers back to the credit system mentioned in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it talks about the effects of credit. It uses the Logical Order rule, as it moves from discussing creditworthiness to the consequences of credit.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives a conclusion about the importance of a balanced credit system. It uses the Cause and Effect Relationship rule, as it shows the result of excessive borrowing.