First sentence: Why this is the first line? This paragraph is the first because it introduces the main topic. It tells us about the researchers and their goal. This is a clear topic sentence. It does not need any other sentence to understand it. Why other lines are not chosen? Paragraph 2 starts with 'They' which is a pronoun. It needs more context to understand who 'they' are. Paragraph 3 starts with 'Initially', which suggests it is not the first idea. Paragraph 4 starts with 'Upon reaching', which also needs more context. All these reasons make them not independent.

Concluding sentence: Why this is the last line? This paragraph is the last because it wraps up the findings. It shows the final result of the investigation. The phrase 'slowed down significantly' gives a clear end to the discussion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of the researchers. The phrase 'They conducted' shows that it is about the same researchers. This is a logical order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it describes what happened during the experiments. The word 'Initially' shows the start of the results from the experiments.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains what happened after the initial increase. The phrase 'Upon reaching a certain point' shows a change in the results.