A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of locomotion. It does not refer to any previous information and sets the stage for the discussion. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with "Long-held assumptions" which is not general enough to stand alone. Paragraph 3 starts with "Similarly" which is a conjunction. Paragraph 4 starts with "But" which is also a conjunction. All these do not meet the criteria for being independent.

Concluding sentence: Why this is the last line? This paragraph concludes the text by summarizing the importance of movement. It reflects on the main theme of locomotion and variation, which ties back to the earlier points.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it provides a broader overview of the topic. It shows that there are assumptions about locomotion that have changed, which relates to the need for understanding locomotion.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it continues the discussion about challenges in locomotion. The word "Similarly" shows that it is related to the previous point about assumptions.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it talks about the importance of movement. It shows that not only do robots struggle, but also how movement affects people, linking the ideas.