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 an aquarium. It uses the rule of Topic Sentence because it gives a broad overview of what is important at the start. It does not refer to any previous information, making it independent. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "Subsequently," which is a conjunction. Paragraph 3 starts with "Once," which is also a conjunction. Paragraph 4 starts with "The next step," which refers back to something not yet mentioned. Paragraph 5 starts with "Finally," which indicates it is not 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 uses the rule of Conclusive Words with the word "Finally," which signals the end of the steps. It summarizes the final action of introducing the fish.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 logically connects with Paragraph 1 because it follows the order of setting up an aquarium. The word "Subsequently" is a connector that shows the next step after selecting the aquarium.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it continues the process of setting up the aquarium. The word "Once" shows that adding a heater comes after the filtration system is installed.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the next step after setting the temperature. The phrase "The next step" indicates a logical order in the process.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it describes what to do after adding plants and decorations. The word "Finally" shows that this is the last step in the process.