A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic of Mars missions. It gives a broad overview of the Mars Pathfinder and the rover Sojourner. It does not refer to any previous information. Why other lines are not chosen? Paragraph 2 starts with "Sojourner" which is a pronoun referring to the rover, so it cannot be independent. Paragraph 3 also starts with "Similarly" which is a conjunction, so it cannot be independent. Paragraph 4 starts with "The only unfortunate thing" which uses the definite article "the" and assumes prior knowledge, so it cannot be independent.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion about the Mars mission. It provides a conclusion about the mission's outcome, highlighting an unfortunate aspect. It does not introduce new information but summarizes a point related to the mission.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the story of Sojourner. It uses the phrase "Sojourner has now been effectively switched off" which shows a logical order from the introduction of the rover to its status.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects to Paragraph 2 because it continues to discuss the lander and its performance. The word "Similarly" shows a logical connection, indicating that both Sojourner and the lander had longer lifetimes than expected.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it shifts the focus to a different aspect of the mission. It introduces a contrast with the phrase "The only unfortunate thing" which shows a different point of view about the mission.