A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about human shoulders. It gives a broad overview of how human shoulders are shaped. It does not need other sentences to understand it. Why other lines are not choosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "This is in contrast," which is a conjunction. Paragraph 3 starts with "The ape shoulder," but it refers to the ape shoulder without introducing it first. Paragraph 4 starts with "On the other hand," which is also a conjunction.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about shoulders. It gives a final point about the advantages of the human shoulder. It summarizes the main ideas about both shoulders.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it uses the word "This is in contrast." This shows that it is comparing human shoulders to ape shoulders. This is a Connector strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it talks about the ape shoulder. It gives more details about the ape shoulder after mentioning it in Paragraph 2. This is a Broader to Specific Strategy.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it starts with "On the other hand." This shows a contrast between the ape shoulder and the human shoulder. This is a Connector strategy.