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 what happens when a candle burns. This is a general statement that does not need other sentences for context. It does not use any pronouns or definite articles that refer to previous ideas. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the criteria for being independent. For example, Paragraph 2 starts with 'As', which is a conjunction. Paragraph 3 uses 'that', which is a pronoun referring to something not yet mentioned. Paragraph 4 starts with 'This', which also refers back to something else.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the main idea of the text. It explains the result of the air movement around the flame. The phrase 'This creates' shows that it is concluding the discussion about the flame.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of the warm air rising. The word 'As' shows a logical order of events. The warm air moving up leads to the cooler air rushing in.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains what happens to the cooler air. The phrase 'When that cooler air is heated' shows a cause and effect relationship. The cooler air rises after being heated.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it describes the result of the rising air. The word 'This' refers to the cycle created by the rising air. It shows how the air movement gives the flame its shape.