A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of how mountains are formed. It is a general statement that does not need another sentence for context. It does not start with a conjunction or a pronoun. It uses an indefinite article 'a' to introduce the concept of mountains being built up. Why other lines are not chosen? Paragraph 2 starts with 'Such mountains' which refers back to a previous idea, so it is not independent. Paragraph 3 starts with 'Underground forces' which is specific and does not stand alone. Paragraph 4 starts with 'Another way' which implies a previous context. Paragraph 5 starts with 'Some blocks' which also refers back to earlier ideas.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it summarizes the ideas about mountain formation. It gives a final detail about how blocks can be lifted or sunk, wrapping up the discussion.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains how the mountains are built up. It follows a logical order where the first paragraph introduces the process, and the second paragraph gives more detail about the forces involved.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it defines the type of mountains being discussed. It uses the connector 'such' to refer back to the mountains mentioned in the previous paragraph.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it introduces another method of mountain formation. The phrase 'Another way' shows a logical continuation of the topic.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 connects with Paragraph 4 because it gives more detail about the blocks mentioned. It continues the discussion about how mountains can form, showing a logical order in the explanation.