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 anthropologists label different ages of man. This is a broad overview of the subject. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with 'When the time comes', which is a phrase that needs context. Paragraph 3 starts with 'Histories of the time', which also needs prior information. Paragraph 4 starts with 'Men and women moved', which is specific and does not introduce the topic. Paragraph 5 starts with 'This situation', which refers back to something not yet mentioned.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion. It explains the overall theme of how the way of life affected people. It 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 topic of anthropologists. It uses the phrase 'When the time comes', which shows a logical order of thought. It suggests that after labeling past ages, anthropologists will look at the twentieth century.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it gives an example of what the histories will say. The phrase 'Histories of the time' shows a logical order, explaining what happened in the twentieth century.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains why people forgot how to use their legs. The logical order is clear as it shows the cause of the problem mentioned in Paragraph 3.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 follows Paragraph 4 because it explains the situation mentioned in Paragraph 4. The phrase 'This situation' refers back to the earlier discussion about how people moved, showing a clear connection.