A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of solid waste management. It gives a broad overview of the issue 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 "This is mainly due to..." which needs context. Paragraph 3 starts with "In addition to..." which also needs prior information. Paragraph 4 starts with "The last reason is..." which refers to something mentioned before.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion by giving the final reason. It uses the phrase "The last reason is..." which signals that it is concluding the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains why solid waste management is a challenge. This follows the Cause and Effect Relationship rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it adds more information about the costs of solid waste management. This follows the Broader to Specific Strategy rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it gives another reason related to the lack of understanding. This follows the Logical Order rule.