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 meteorites found by Mars Rovers. It does not need another sentence for context and does not use any pronouns or definite articles that refer to previous information. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the criteria for being independent. For example, Paragraph 2 starts with 'On Earth,' which refers to a specific place and does not stand alone. Paragraph 3 asks a question and does not provide information. Paragraph 4 uses 'the answer,' which assumes prior knowledge. Paragraph 5 starts with 'The surface of Earth,' which also assumes prior context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the main points about the environment on Earth and its effect on meteorites. It brings together the earlier points about Mars and Earth, providing a clear conclusion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it provides a comparison. The phrase 'On Earth' shows a contrast between Mars and Earth. This is a logical order because it moves from the discovery of meteorites on Mars to how humans find them on Earth.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it raises a question about the abundance of meteorites on Mars. This question follows the information about Earth, making it a logical progression.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it directly answers the question posed in Paragraph 3. This shows a clear logical order from question to answer.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects to Paragraph 4 because it provides specific details about the environment on Earth. This follows the general statement about the environment in Paragraph 4, showing a broader to specific strategy.