A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of computer users and their health problems. It does not need another sentence for context. It gives a broad overview of the issue without going into details. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'They', which is a pronoun that refers back to something. Paragraph 3 starts with 'Thus', which is a conjunction. Paragraph 4 starts with 'But', which is also a conjunction. These words show that they need more context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the difficulties in treating repetitive stress injuries. It summarizes the main points and gives a final thought on the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of computer users. It explains what happens when they ignore warnings. This shows a logical order of cause and effect.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shows the result of the problems mentioned. After explaining the injuries, it talks about how people look for relief. This is a broader to specific strategy.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it talks about the challenges of finding cures. It shows a contrast between searching for relief and the difficulty in finding effective treatments.