First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about flies. It gives a broad overview of how people see flies. It does not need other sentences to understand it. It does not use any pronouns or definite articles that refer to previous ideas. Why other lines are not chosen? The other paragraphs are not chosen because they do not meet the rules for being independent. For example, Paragraph 2 starts with "Although," which is a conjunction. Paragraph 3 starts with "Some of these," which uses a pronoun. Paragraph 4 starts with "These are recyclers," which uses the definite article "the." 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 about flies. It explains their role as recyclers and how they help plants. This gives a clear conclusion to the topic of flies.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it uses the connector "Although" to show a contrast. It talks about the harmful side of some flies, which relates to how people think of flies in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it continues the idea about flies. It shows that even flies that seem unattractive are important. This is a logical order from discussing harmful flies to important flies.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains why some flies are important. It uses the term "These are recyclers" to show what flies do, which is a specific detail following the general idea in Paragraph 3.