A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of cup handle design. It clearly states the importance of the handle for comfort and functionality. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'A well-designed handle', which refers back to the handle and is not independent. Paragraph 3 starts with 'On the contrary', which is a transitional phrase and does not stand alone. Paragraph 4 starts with 'Therefore', which also indicates a conclusion and cannot be first.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about handle design. It uses the rule of Conclusive Words with 'Therefore', showing that it concludes the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it gives more details about what a well-designed handle does. This follows the Broader to Specific Strategy. The first sentence talks about the importance of the handle, and the second sentence explains how it helps.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shows the opposite idea. It uses the Connector strategy with 'On the contrary'. This shows a contrast between well-designed and poorly designed handles.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it gives a conclusion about handle design. It uses the Conclusive Words strategy with 'Therefore', which shows that it is a summary of the previous points.