Meaning: 'Measuring' refers to the process of determining the size, amount, or degree of something, in this case, the levels of carbon dioxide in atmospheres.

Wrong options:

• embedding: Embedding implies inserting something within something else, which does not fit the context of detecting levels.
• promising: Promising suggests potential or hope, but does not convey the action of quantifying or assessing levels.
• confusing: Confusing indicates a lack of clarity, which is not relevant to the action of detection in this context.

Correct answer: because

Explanation: The word 'because' introduces a causal relationship, explaining why planets with large oceans have lower CO2 levels. It fits grammatically as a conjunction that connects the two clauses logically.

Meaning: 'Because' means for the reason that; it introduces an explanation for the preceding statement about CO2 levels.

Wrong options:

• so far: So far indicates a time frame and does not provide a causal explanation, making it contextually inappropriate.
• due to: Due to is a preposition that requires a noun phrase, not fitting the grammatical structure of the sentence as it needs a conjunction.
• ever since: Ever since indicates a time period and does not establish a causal relationship, thus not suitable for this context.

Correct answer: presence

Explanation: The word 'presence' fits grammatically as a noun that indicates existence or occurrence, which is what the researchers are suggesting could be indicated by lower CO2 levels. It aligns contextually with the idea of liquid water or life being present on a planet.

Meaning: 'Presence' refers to the state of being present or existing in a place, in this case, indicating the existence of liquid water or life.

Wrong options:

• engagement: Engagement refers to involvement or participation, which does not relate to the existence of water or life.
• joy: Joy is an emotion and does not fit the context of scientific detection or existence.
• hint: Hint suggests a suggestion or clue, which is less definitive than 'presence' and does not convey the same level of certainty.

Correct answer: complement

Explanation: The word 'complement' fits grammatically as a verb that means to enhance or complete something, which is appropriate in the context of adding to other strategies. It aligns contextually with the idea of this method working alongside other strategies.

Meaning: 'Complement' means to complete or enhance something by providing additional features or elements.

Wrong options:

• oppress: Oppress means to keep someone in subservience, which is not relevant to the context of enhancing strategies.
• seduce: Seduce implies enticing someone into a desired action, which does not fit the context of scientific methods.
• consist: Consist means to be composed of, which does not convey the idea of enhancing or adding to other strategies.

Correct answer: step

Explanation: The word 'step' fits grammatically as a noun that indicates a stage in a process, which is appropriate in the context of discovering life. It aligns contextually with the idea of finding liquid water being a crucial part of the process of discovery.

Meaning: 'Step' refers to a stage or action taken in a process, indicating that finding liquid water is a necessary phase in the search for life.

Wrong options:

• career: Career refers to a profession or occupation, which does not fit the context of a process in scientific discovery.
• finance: Finance relates to money management, which is irrelevant to the context of discovering life in the universe.
• setting: Setting refers to the environment or context, which does not convey the idea of a necessary action or stage in the discovery process.