First sentence: Why this is the first line? This paragraph is the first sentence because it gives a clear definition of what a human population is. It introduces the main topic without needing other sentences for context. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'We talk about...' which is not independent because it refers back to the topic. Paragraph 3 starts with 'Governments count...' which is not independent because it assumes knowledge of the population. Paragraph 4 starts with 'The population...' which uses 'the' and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes how we can understand population changes. It wraps up the ideas presented in the earlier paragraphs, following the Summary Style rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it gives examples of where we can find human populations. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains how governments measure the population mentioned in Paragraph 2. This follows the Logical Order rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains what happens between census counts. This follows the Cause and Effect Relationship rule.