A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of carbon in living things. It does not need other sentences for context. It gives a broad overview of carbon's role in nature. Why other lines are not choosen? Paragraph 2 starts with "Once plants and animals die," which needs context. Paragraph 3 starts with "When humans burn fossil fuels," which also needs prior information. Paragraph 4 starts with "Humans have burned so much fossil fuel," which refers back to earlier ideas.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the main idea of the text. It shows the result of the actions discussed in the earlier paragraphs. It wraps up the topic of carbon and its impact on the earth.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of carbon. It explains what happens to carbon after living things die. This shows a logical order from living things to what happens after they die.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it talks about what happens when humans use fossil fuels made from carbon. This shows a clear flow from the creation of fossil fuels to their use.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains the effect of burning fossil fuels. It shows the cause (burning fossil fuels) and the effect (global warming), which is a logical order.