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 the cold summer in 1816. It does not need any other sentence for context. It gives a broad overview of what the text will discuss. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'The year before,' which needs context from a previous sentence. Paragraph 3 starts with 'In such volcanic explosions,' which also needs prior information. Paragraph 4 starts with 'This can have,' which refers back to something mentioned before.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the ideas presented in the text. It explains the final effect of the volcanic eruption and the cold summer. It gives a clear conclusion to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains why there was a cold summer. The phrase 'the year before' shows a chronological order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains what happens during volcanic eruptions. The phrase 'in such volcanic explosions' shows a logical order.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains the effect of the dust particles. The word 'this' refers back to the dust particles mentioned in Paragraph 3, showing a noun pronoun rule.