First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main idea. It talks about a single ring of ripples from one stone. This 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 "But" which is a conjunction. Paragraph 3 uses "Several rings" which is not general. Paragraph 4 uses "It" which is a pronoun. 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 main idea. It gives the overall theme of the text. It shows what the previous paragraphs were discussing.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the idea of ripples. It shows a broader idea of how multiple stones create multiple rings. This is a logical order from one ring to several rings.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it uses the word "But". This shows a contrast. It explains that even with many stones, only one ring is made if they are thrown quickly. This is a cause and effect relationship.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains the main idea of the previous paragraph. It tells us that the topic is about 'frequency'. This is a logical order from a specific example to the general topic.