First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of pollution. It clearly states what pollution is without needing any other information. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'They', which is a pronoun and needs more context. Paragraph 3 starts with 'Pollutants', which is not a general statement. Paragraph 4 starts with 'These', which also needs prior context. All these do not stand alone.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the main idea about pollutants. It wraps up the discussion by explaining the effects of pollutants, which follows the Summary Style rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains how pollution can happen. This follows the Logical Order rule, as it continues the idea of pollution with examples of human activity.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shows another type of pollutants. This follows the Broader to Specific Strategy, as it moves from human-made pollutants to natural ones.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it defines what pollutants are. This follows the Topic Example rule, as it gives more information about pollutants.