A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of visiting Mars. It does not need another sentence for context. It is a general statement that sets the theme for the following paragraphs. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with "That's where" which refers back to the previous idea. Paragraph 3 starts with "For that reason" which also needs prior context. Paragraph 4 starts with "They're hopeful" which refers to the previous information. All these sentences need context from earlier sentences.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion. It summarizes the hopes of the microbiologists and reflects the main theme of the text about Mars and biology.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains what you will find in Turkey. The phrase "That's where" shows a link to the previous idea about visiting Turkey.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it explains why microbiologists are studying the lakes. The phrase "For that reason" shows a cause and effect relationship.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains what microbiologists hope to learn from their studies. The word "They're hopeful" shows the continuation of the idea about the microbiologists.