A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of arsenic as a big environmental problem. It does not need other sentences to understand it. It gives a broad overview of the issue. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the criteria for being independent. For example, Paragraph 2 starts with 'Naturally' which is not a general statement. Paragraph 3 starts with 'It' which is a pronoun and needs context. Paragraph 4 starts with 'Natural' which is also not a general statement.

Concluding sentence: Why this is the last line? This paragraph is the last line because it sums up the problem of arsenic pollution. It gives a final detail about how many countries are affected and the seriousness of the situation.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains how arsenic is dangerous. The phrase 'Naturally occurring arsenic leaches' shows a cause of the problem mentioned in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it describes the properties of arsenic. The phrase 'It is colourless and odourless' continues the idea of why people might not notice arsenic in water.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it shows the wider impact of arsenic pollution. The mention of '21 countries' gives a broader view of the problem that started in Paragraph 3.