A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of the text. It talks about the UK leaving the EU and connects to the idea of an earlier 'Brexit'. This is an introductory statement that sets the stage for the following paragraphs. It does not use any pronouns or definite articles that refer to previous ideas. Why other lines are not chosen? Paragraph 2 starts with 'They', which is a pronoun and needs context. Paragraph 3 starts with 'The researchers', which also uses a definite article and assumes prior knowledge. Paragraph 4 starts with 'The scars', which again uses a definite article and assumes prior context.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion. It summarizes the effects of the events mentioned in the previous paragraphs. It provides a clear ending to the topic of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the idea of the UK and its history. The phrase 'they have worked out' refers back to the scientists mentioned in Paragraph 1. This shows a logical order of thought.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 explains the reason for the destruction mentioned in Paragraph 2. It gives details about what caused the land to be destroyed. This shows a cause and effect relationship.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 concludes the ideas in Paragraph 3 by showing the result of the events discussed. The phrase 'the scars of these events' refers back to the destruction mentioned before, showing a logical order.