A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text, which is about planets in our solar system. It does not refer to any previous information. It sets the stage for the discussion about Earth and life. Why other lines are not choosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'Mars' which refers to a specific planet and needs context. Paragraph 3 starts with 'But' which is a conjunction. Paragraph 4 uses 'There are' which is not a general statement. Paragraph 5 starts with 'So' which also indicates a continuation of thought.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about stars. It gives a final estimate of the number of stars, which is a summary of the earlier points made in the text.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the discussion about planets. It gives examples of why Earth is special compared to other planets. This shows a logical order from a general statement about Earth to specific details about other planets.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shifts from discussing our solar system to the possibility of other planets around other stars. The word 'But' shows a contrast and introduces a new idea about the vast number of stars.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it provides specific numbers about stars and galaxies. This shows a logical order from the idea of many stars to the details about how many stars exist.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it summarizes the information about the number of stars. The word 'So' indicates that this is a conclusion based on the previous information.