First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic. It talks about Thomas Midgley and his work on 'engine knock'. This is a general statement that sets the stage for what follows. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with 'At that time', which refers to a previous idea, so it cannot be first. Paragraph 3 starts with 'He then', which uses a pronoun that refers back to Midgley, so it cannot be first. Paragraph 4 starts with 'These two chemicals', which also refers back to earlier information, so it cannot be first.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion. It summarizes the negative effects of the chemicals. The phrase 'until their true impact was finally identified' shows a conclusion to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the story of Midgley. The phrase 'At that time' shows the time context after introducing Midgley. This is a logical order of ideas.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it shows what Midgley did next. The word 'He then' indicates a new action after solving the noise problem. This shows a logical order of events.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it talks about the chemicals that were used. The phrase 'These two chemicals' refers back to the toxic chemicals mentioned in Paragraph 3. This shows a clear connection.