First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of chemical pest control. It gives a broad overview of the history of pest control, which sets the stage for the following paragraphs. It does not use any pronouns or definite articles that would need prior context. Why other lines are not chosen? Paragraph 2 starts with "The Rig Veda," which uses the definite article "the" and assumes prior knowledge. Paragraph 3 starts with "And," which is a conjunction and does not stand alone. Paragraph 4 starts with "But," which is also a conjunction and cannot be independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion on chemical pest control. It shows how pest control evolved into a preferred method with modern techniques. The phrase "method of choice" indicates a conclusion to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of pest control. It provides a specific example of pest control from a historical text, which follows the general statement in Paragraph 1. This shows a Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it adds more examples of ancient cultures that used pest control. The use of "And" shows a continuation of thought, linking the ideas of different cultures using pest control.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 by showing a change in pest control methods. The word "But" indicates a contrast between the ancient methods and the modern methods, which is a logical progression.