First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text. It tells us that the footprints will belong to a gecko. This is a clear topic sentence that sets the stage for the following information. It does not refer to any previous context or use specific articles that need prior knowledge. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'If Kellar Autumn...' which is a conditional phrase and needs more context. Paragraph 3 starts with 'A gecko can...' which is specific and assumes prior knowledge. Paragraph 4 uses 'If Kellar Autumn...' which also needs context. All these sentences are not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it concludes the text. It wraps up the ideas by stating that the footprints will not be human, but from a gecko. This gives a final thought on the topic introduced in the first paragraph.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains why the footprints belong to a gecko. It uses the connector 'and' to show that the gecko's sticking power is important for the next generation of robots.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it gives an example of the gecko's abilities. It shows how the gecko can use its sticking power to climb walls, which supports the idea in Paragraph 2.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it introduces Kellar Autumn's idea about using gecko abilities for Mars. It shows the logical order of how the gecko's skills can lead to the idea of footprints on Mars.