A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of setting up a home aquarium. It does not need another sentence for context. It clearly shows the main idea without referring to anything before it. Why other lines are not chosen? Paragraph 2 starts with "The selection" which uses the definite article "the". This means it refers to something already known, so it is not independent. Paragraph 3 starts with "Following" which is a conjunction, so it cannot be the first sentence. Paragraph 4 starts with "Once" which is also a conjunction, so it cannot be the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the process of setting up the aquarium. It shows the final step of adding fish and plants, which is the conclusion of the aquarium setup.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of setting up an aquarium. The first paragraph talks about deciding to set up the aquarium, and the second paragraph talks about the important choice of tank and equipment. This shows a logical order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it follows the idea of setting up the equipment. After choosing the right tank and equipment, the next step is the cycling process. This shows a clear logical order.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it continues the story after the cycling process. Once the setup is done, the next step is adding fish and plants. This shows a clear logical order.