First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of fossil fuels. It gives a broad overview of their importance in industrial development. It does not use any pronouns or definite articles that refer to previous information. Why other lines are not chosen? Paragraph 2 starts with 'Their', which is a pronoun that refers back to fossil fuels, so it cannot be independent. Paragraph 3 starts with 'However', which is a conjunction and shows a contrast, so it cannot be independent. Paragraph 4 starts with 'Thus', which is also a conjunction and shows a conclusion, so it cannot be independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion. It shows the conclusion of the ideas presented in the previous paragraphs. It uses the conclusive word 'Thus' to indicate that it is summarizing the main points.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 explains why fossil fuels are important. It uses the Broader to Specific Strategy. The first paragraph gives a general idea about fossil fuels, and the second paragraph gives specific reasons for their preference.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 shows a contrast to the benefits mentioned in Paragraph 2. It uses the Contrast rule. The second paragraph talks about the advantages of fossil fuels, and the third paragraph explains the negative effects of using them.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 shows the result of the issues mentioned in Paragraph 3. It uses the Cause and Effect Relationship rule. The third paragraph discusses the problems caused by fossil fuels, and the fourth paragraph explains how this leads to the search for better energy options.