First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic. It tells us what the scientists did. It does not need another sentence to understand it. It does not start with a conjunction or a pronoun. It uses an indefinite article 'a' to introduce the group of students. Why other lines are not chosen? Paragraph 2 starts with 'The teacher', which uses a definite article 'the' and assumes prior knowledge. Paragraph 3 starts with 'In reality', which is a transitional phrase. Paragraph 4 starts with 'However', which is also a conjunction. All these reasons make them not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the story. It shows the final outcome of the students' performance. It gives a conclusion to the earlier points.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the story. The teacher is reacting to what the scientists said. This shows a logical order.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it gives the truth about the students. It shows a cause and effect relationship. The teacher's assumption is corrected.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the result of the random selection. It uses 'However' to show a contrast between the selection process and the students' performance.