First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic. It gives a broad overview of the risks Earth faces from cosmic events. It does not need any other sentence 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 "In 1991" which is a specific time reference, so it does not stand alone. Paragraph 3 starts with "The group" which uses a pronoun that refers back to a previous idea. Paragraph 4 starts with "Although" which is a conjunction, so it cannot be the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion. It summarizes the findings about the risk of large impacts. It uses the phrase "careful study shows" which indicates a conclusion to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of cosmic events. The phrase "In 1991" shows a chronological order, telling us when Congress took action about the hazards mentioned in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 logically because it gives the result of the investigation mentioned in Paragraph 2. It shows a logical order where the study's conclusion comes after the request for the study.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it discusses the risk of impacts. It uses the word "Although" to show a contrast between the risk of impacts and the conclusion that the risk is small. This shows a cause and effect relationship.