First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of solar energy reaching Earth. It does not refer to any previous information and sets the stage for the following sentences. It uses a broad overview to explain what happens to the Sun's energy. Why other lines are not chosen? Paragraph 2 starts with 'This radiation,' which refers back to the energy mentioned in Paragraph 1, so it cannot be first. Paragraph 3 starts with 'However,' which is a conjunction and shows a continuation of thought. Paragraph 4 uses 'A greenhouse gas,' which assumes prior knowledge of greenhouse gases, making it dependent. Paragraph 5 starts with 'If any difference,' which is not a general statement and needs context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about greenhouse gases and their effects. It introduces a potential consequence of the previous information, making it a fitting conclusion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 logically connects with Paragraph 1 because it continues the idea of solar energy. The phrase 'This radiation' refers back to the energy from the Sun, showing a clear flow of ideas.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it discusses what happens to the infrared radiation. The word 'However' shows a contrast, indicating that while some radiation goes into space, some is absorbed.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains what greenhouse gases do. The phrase 'A greenhouse gas' introduces a specific topic that relates to the previous discussion about absorption of radiation.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it discusses the effects of greenhouse gases. The phrase 'If any difference in this process' suggests that changes in how greenhouse gases work can affect climate change.