First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of space exploration. It gives a broad overview of the benefits of space exploration without going into details. This fits the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with "Recent missions" which is specific and needs context. Paragraph 3 starts with "However" which is a conjunction. Paragraph 4 starts with "The development" which uses the definite article and assumes prior knowledge. Paragraph 5 starts with "The future" which also uses the definite article.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about space exploration. It gives a final thought about the impact of space exploration on humanity. This fits the rule of Conclusive Words.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of space exploration. It provides specific examples of discoveries from recent missions. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shifts the focus to another important aspect of space exploration, which is the development of reusable rockets. This shows a Logical Order of ideas.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it introduces a contrasting idea. It shows that while space travel is becoming cheaper, there are ethical concerns. This uses the Contrast rule.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it looks ahead to the future of space exploration. It suggests that the ethical questions will shape what happens next. This follows the Logical Order of ideas.