A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic. It talks about the decline in marriage rates and the increase in divorce rates. This is a broad overview of the subject. It does not refer to any previous information. Why other lines are not chosen? Paragraph 2 starts with "In 1986," which is a specific time and does not introduce the main topic. Paragraph 3 starts with "Conversely," which shows a contrast and needs context. Paragraph 4 starts with "At the same time," which also needs context. All these sentences do not stand alone as they refer to specific details or need prior information.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion about changes in marriage and divorce rates. It provides final details about the divorce rates and widowed population, which summarizes the overall theme of the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 gives specific details about the decline mentioned in Paragraph 1. It follows the Broader to Specific Strategy. The first paragraph talks about the general decline in marriage, and the second paragraph provides specific numbers.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 shows a contrast to the decline in marriage in Paragraph 2. It uses the Connector strategy with the word "Conversely" to show the opposite trend.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 continues the discussion about changes in marital status. It uses the Logical Order strategy, as it shows another change in the population's marital status at the same time.