A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic. It tells us about a new system for 3-D printing buildings. This is a broad overview of what the text will discuss. It does not use any words that need prior context. Why other lines are not chosen? Paragraph 2 starts with "Structure" which is not independent because it refers back to the system in Paragraph 1. Paragraph 3 starts with "Even" which is a conjunction and needs context. Paragraph 4 starts with "Ultimately" which is also a conjunction and does not stand alone.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the main ideas. It shows the final result of using the new system. It uses the phrase "Ultimately" which signals a conclusion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the idea of the system. It uses the phrase "this system" which refers back to the system mentioned in Paragraph 1. This is a clear example of the Noun Pronoun Rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives more details about the structure. It explains how the structure can be changed. This shows a Broader to Specific Strategy.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it talks about the results of the changes mentioned in Paragraph 3. It shows how these changes can help in building new types of buildings. This is a Logical Order.