A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic, which is Leucine. It gives a broad overview of what Leucine is and its importance. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "Animals' protein..." which needs context. Paragraph 3 starts with "Plants' protein's..." which also needs context. Paragraph 4 starts with "However..." which shows it is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion about Leucine. It introduces exceptions, which gives a final thought to the topic. This follows the rule of Conclusive Words.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of Leucine. The Logical Order is clear as it moves from the general idea of Leucine to a specific example of its amount in animal protein.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 logically follows Paragraph 2 because it gives a comparison. The Broader to Specific Strategy is used here, as it first states the amount in animals and then explains the lower amount in plants.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it introduces exceptions to the previous statement. The Connector Strategy is used here with the word "However," showing a contrast.