A.I. Explanation:

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 the establishment of the United Nations. It gives a broad overview of the topic without going into specifics. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, Paragraph 2 starts with 'At the very beginning,' which makes it dependent on prior context. Paragraph 3 starts with 'This number,' which refers back to a number not yet mentioned. Paragraph 4 starts with 'Consequently,' which shows it is linked to previous information.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the main ideas of the text. It summarizes the importance of the United Nations after its establishment and growth, following the rule of Summary Style.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the story of the United Nations. The phrase 'At the very beginning' shows a logical order of events, explaining what happened right after the establishment.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it talks about the growth of the member states. The phrase 'This number' refers back to the 51 member states mentioned in Paragraph 2, following the Noun Pronoun Rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains the impact of the growth in member states. The word 'Consequently' shows a cause and effect relationship, indicating that the growth led to a more significant role for the United Nations.