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. It tells us what bitcoin is. This follows the rule of Topic Sentence. It does not need any other sentence for context. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, 'That's to say you need to download mobile phone app to use such money.' starts with 'That's to say', which is a conjunction. 'With above done it generates a bitcoin address.' uses 'With', which is also a conjunction. 'This address can only be used once.' is a specific statement and does not introduce the topic.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the information about the bitcoin address. It gives a final detail about the address. This follows the Conclusive Words rule, as it provides a clear end to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains what you need to do with bitcoin. This follows the Broader to Specific Strategy. The first sentence gives a general idea about bitcoin, and the second sentence gives specific information about using it.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it shows what happens after you download the app. This follows the Logical Order rule. The second sentence talks about downloading, and the third talks about generating an address.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains what the generated address is. This follows the Cause and Effect Relationship rule. The third sentence talks about generating an address, and the fourth tells us about the address.