Meaning: 'But' is used to introduce a statement that contrasts with or contradicts something that has been said previously, indicating a shift in perspective.

Wrong options:

• while: While suggests a simultaneous occurrence or contrast, which does not fit the context of explaining reasons for the pause in moon landings.
• hence: Hence implies a conclusion drawn from the previous statement, which does not logically connect with the idea of contrasting reasons for the pause.
• so: So indicates a result or consequence, which does not align with the need to contrast the reasons for the pause in moon landings.

Correct answer: clinched

Explanation: The verb 'clinched' fits grammatically as it indicates that the Apollo missions secured or achieved a significant victory for the USA in the space race. This aligns with the context of the sentence, which discusses the urgency that faded after the moon was reached.

Meaning: 'Clinched' means to secure or finalize something, often used in the context of achieving a victory or success.

Wrong options:

• climbed: Climbed suggests a physical ascent, which does not accurately describe the achievement of winning the space race.
• ached: Ached implies a feeling of pain or longing, which is not relevant to the context of achieving a victory in the space race.
• pinched: Pinched typically refers to a physical action of squeezing or a financial constraint, neither of which fits the context of winning.

Correct answer: prohibitive

Explanation: The adjective 'prohibitive' accurately describes the costs of moon missions as being so high that they prevent further action. This fits grammatically and contextually, as it highlights the financial burden associated with continued lunar exploration.

Meaning: 'Prohibitive' means that something is so high in cost that it discourages or prevents action, particularly in financial contexts.

Wrong options:

• supplementary: Supplementary suggests something additional, which does not convey the idea of costs being a barrier to further missions.
• differential: Differential refers to a difference or distinction, which does not apply to the context of costs being a burden.
• professional: Professional relates to a career or occupation, which does not fit the context of describing the nature of costs.

Correct answer: practical

Explanation: The adjective 'practical' fits well as it describes the space shuttle program and robotic explorers as being sensible and feasible options compared to manned lunar missions. This aligns with the context of the sentence, which discusses appealing alternatives to moon missions.

Meaning: 'Practical' means being sensible and realistic, particularly in terms of usefulness and feasibility.

Wrong options:

• conceptual: Conceptual relates to ideas or theories, which does not fit the context of discussing the feasibility of missions.
• terminal: Terminal suggests an end or limit, which does not apply to the context of describing the nature of space missions.
• preliminary: Preliminary refers to something that comes before the main event, which does not accurately describe the nature of the missions being discussed.