A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of science writing. It does not refer to any previous information and sets the stage for the discussion. It uses an indefinite article 'a' to introduce the general idea of a formula for science writing. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'Turning out even 700 words...' which is not a general statement. Paragraph 3 starts with 'For research students...' which refers to a specific group. Paragraph 4 starts with 'But if...' which is a conjunction and needs prior context.

Concluding sentence: Why this is the last line? This paragraph is the last line because it concludes the discussion about writing and fame. It uses the phrase 'you can hope' which suggests a final thought or reflection on the topic. It wraps up the ideas presented in the previous paragraphs by connecting personal success in writing to the hope of being recognized.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the discussion about writing. The first paragraph talks about the lack of a formula for science writing, and the second paragraph discusses the challenge of writing itself. This shows a logical order from the general idea of writing to the specific challenge of writing 700 words.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it shifts focus to research students and their interest in science writing. The second paragraph talks about the writing process, and the third paragraph explains who finds science writing interesting. This shows a logical order from the general writing challenge to the specific audience interested in science.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it introduces a contrasting idea. The third paragraph discusses research students and their interests, while the fourth paragraph talks about a different perspective on writing, suggesting that some people may become famous later. This shows a contrast between the focus on students and the idea of becoming a best-seller.