A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It talks about how people are living longer, which sets the stage for the following information. It does not use any words that need prior context. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'Today', which needs context. Paragraph 3 uses 'By 2030', which also needs context. Paragraph 4 starts with 'At this time', needing prior information. Paragraph 5 starts with 'By 2050', which again needs context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it gives a final prediction about the future. It wraps up the information by showing how the population will change by 2050. It summarizes the main points discussed in the earlier paragraphs.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of living longer. The word 'Today' shows that it is talking about the present situation, which follows the general statement in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it gives specific information about the future. It shows how many people will be older in the coming years, following the idea of living longer.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains what will happen at that time. It gives more details about the increase in the older population, following the prediction in Paragraph 3.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it continues the discussion about the older population. It shows a future prediction that builds on the information given in Paragraph 4.