A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of beef and its nutrients. It does not refer to any previous information. It sets the stage for the discussion about beef. Why other lines are not chosen? Paragraph 2 starts with 'These nutrients', which refers back to the nutrients mentioned in Paragraph 1, so it cannot be first. Paragraph 3 starts with 'However', which is a conjunction and shows a contrast, so it cannot be first. Paragraph 4 starts with 'Therefore', which also shows a conclusion and cannot be first.

Concluding sentence: Why this is the last line? This paragraph is the last line because it gives a conclusion based on the previous information. It wraps up the discussion by providing a recommendation, which is a clear ending to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): This pair connects because Paragraph 2 explains why the nutrients in Paragraph 1 are important. This follows the Broader to Specific Strategy, where the first paragraph gives a general idea about nutrients, and the second gives specific information about their importance.
• Pair 2 (Paragraph 2 → Paragraph 3): This pair connects because Paragraph 3 introduces a contrast to the benefits mentioned in Paragraph 2. The word 'However' shows that there is a different side to the topic, following the Contrast rule.
• Pair 3 (Paragraph 3 → Paragraph 4): This pair connects because Paragraph 4 gives a recommendation based on the issues mentioned in Paragraph 3. The word 'Therefore' shows that the recommendation is a result of the problems discussed, following the Cause and Effect Relationship rule.