A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of dietary supplements. It does not refer to any previous information and sets the stage for the discussion. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'Their' which is a pronoun, so it cannot be independent. Paragraph 3 starts with 'But', which is a conjunction, so it cannot be independent. Paragraph 4 starts with 'The', which is a definite article and assumes prior knowledge, so it cannot be independent. Paragraph 5 starts with 'It', which is also a pronoun, so it cannot be independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the dangers of dietary supplements. It gives a final thought on the risks involved. This follows the rule of Conclusive Words.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of dietary supplements. It explains what these supplements often contain. This follows the rule of Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it shows a problem with the products mentioned in Paragraph 2. It explains that there are hidden dangers. This follows the rule of Cause and Effect Relationship.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it gives a specific example of a prohibited substance mentioned in Paragraph 3. This follows the rule of Topic Example.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it explains the history of the drug sibutramine. It shows why it is important to know about this substance. This follows the rule of Logical Order.