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 gives a broad overview of what to do first without needing any prior context. It does not use any pronouns or definite articles that refer to previous information. Why other lines are not chosen? Paragraph 2 starts with "After adding the water," which needs context from a previous sentence. Paragraph 3 starts with "Once the tank is in place," which also needs prior information. Paragraph 4 starts with "Finally," which shows 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 word "Finally," which signals the conclusion of the steps.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it follows the logical order of steps. After choosing the tank and adding water, it is important to treat the water.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it continues the logical order. After treating the water, the next step is to install the filtration system.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the final step. After setting up the tank and filtration, you can introduce the fish.