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 gives a broad overview of what will be discussed. It does not refer to any previous information. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, 'Antibiotics, for example...' starts with a specific example and needs context. 'Some drugs are so strong...' uses a pronoun 'they' that refers back to drugs. 'All drugs used to be made...' is a specific statement that needs a general introduction. 'We now know that many of these...' refers to previous information about drugs.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion. It summarizes the main points about drugs and their use. It gives a clear conclusion to the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it gives specific examples of the types of drugs mentioned in Paragraph 1. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it talks about the strength of some drugs. This shows a Logical Order where the discussion moves from examples to a specific point about drug strength.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shifts the focus from the strength of drugs to their origins. This shows a change in topic but keeps the discussion about drugs.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it reflects on the information about drugs made from plants and animals. It concludes the discussion by stating what we know now about these drugs.