A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of currency. It is a general statement that does not need other sentences for context. It does not use any pronouns or definite articles that refer to previous ideas. Why other lines are not chosen? Paragraph 2 starts with "In short" which is a transitional phrase, so it is not independent. Paragraph 3 starts with "In short" and uses a pronoun "it" which refers back to currency, so it is not independent. Paragraph 4 starts with "In the 21st century" which suggests a specific time and context, so it is not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it provides specific details about virtual currencies. It wraps up the discussion by explaining what virtual currencies are and how they work.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains what currency is. It uses the Connector strategy by starting with "In short" to summarize the idea of currency.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it introduces a new type of currency. It follows the Broader to Specific Strategy, as it moves from the general idea of money to the specific idea of virtual currency.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it gives details about virtual currencies. It uses the Logical Order strategy, as it follows the introduction of virtual currency with specific information about it.