First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic, which is the jet stream. It gives a broad overview of what the jet stream is without going into details. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'They', which is a pronoun and needs context. Paragraph 3 starts with 'Instead', which is a conjunction and does not stand alone. Paragraph 4 starts with 'Since', which is also a conjunction and cannot be independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about the jet stream and its effects on aircraft. It summarizes the main points and shows the overall impact of the jet stream.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains why the jet stream exists. This follows the Cause and Effect Relationship rule.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it describes how the jet stream behaves as a result of the temperature differences. This follows the Logical Order rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it explains how the jet stream affects aircraft. This follows the Cause and Effect Relationship rule.