A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about the sea turtle's shell. It is a general statement that does not need other sentences for context. It does not start with a conjunction or use pronouns. Why other lines are not chosen? Paragraph 2 starts with "A series" which is not independent. Paragraph 3 starts with "The nerves" which uses a definite article and assumes prior knowledge. Paragraph 4 starts with "In addition" which is a conjunction and does not stand alone.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about sea turtles and touch. It provides an example of the observations made by researchers, which concludes the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains how the shell senses touch. This follows the Broader to Specific Strategy, where the first sentence gives a general idea and the second provides more details.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it talks about the nerves mentioned in the second paragraph. This follows the Logical Order rule, as it continues the explanation about the nerves and their function.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it gives an example of how sea turtles respond to touch. This follows the Topic Example rule, where the fourth paragraph provides an example related to the topic of touch.