Meaning: 'Asteroids' refers to small rocky bodies orbiting the sun, which are believed to have collided with Earth in its early history, potentially delivering water.

Wrong options:

• Goitres: 'Goitres' refers to an enlargement of the thyroid gland and is unrelated to the context of water sources.
• Groynes: 'Groynes' are structures built to protect the shoreline from erosion, which does not relate to the origin of Earth's water.
• Anthropoids: 'Anthropoids' refers to a group of primates, which is irrelevant in the context of discussing celestial bodies.

Correct answer: provided

Explanation: The verb 'provided' correctly fits the past tense context of the sentence, indicating that comets were thought to have given a significant amount of water to the oceans. It maintains grammatical consistency with the subject and the overall narrative.

Meaning: 'Provided' means to supply or furnish something, in this case, suggesting that comets were believed to have supplied water to Earth.

Wrong options:

• unrivalled: 'Unrivalled' is an adjective meaning unmatched, which does not fit grammatically as a verb in this context.
• hybrid: 'Hybrid' is a noun or adjective referring to a mix of different elements, which does not fit the grammatical structure needed here.
• diapered: 'Diapered' is a past participle form of 'diaper,' which is unrelated and does not fit the context.

Correct answer: signatures

Explanation: The term 'signatures' fits as a plural noun that refers to distinctive chemical markers. It aligns grammatically and contextually with the sentence, which discusses the chemical composition of comets compared to oceans.

Meaning: 'Signatures' in this context refers to unique chemical compositions or characteristics that can identify the source of a substance.

Wrong options:

• sledgehammers: 'Sledgehammers' are heavy tools used for demolition, which is irrelevant to the context of chemical analysis.
• syllables: 'Syllables' refer to units of sound in language, which does not relate to chemical composition.
• reciprocals: 'Reciprocals' are mathematical terms referring to the inverse of a number, which does not fit the context of chemical signatures.

Correct answer: hydrogen

Explanation: The word 'hydrogen' is a noun that fits grammatically as the subject of the phrase, referring to a chemical element. It is contextually appropriate as it relates to the discussion of water's composition.

Meaning: 'Hydrogen' is a chemical element that is a key component of water (H2O), making it relevant to the discussion of ocean composition.

Wrong options:

• cyclotron: 'Cyclotron' is a type of particle accelerator, which is unrelated to the context of chemical elements in water.
• niacin: 'Niacin' is a vitamin (B3) and does not relate to the chemical composition of water.
• lithesome: 'Lithesome' is an adjective meaning graceful or flexible, which does not fit the context of discussing chemical elements.

Correct answer: planetary

Explanation: The adjective 'planetary' fits grammatically as it describes the type of science meeting. It is contextually relevant as it relates to the study of celestial bodies and their origins.

Meaning: 'Planetary' refers to anything related to planets, indicating that the meeting was focused on topics concerning planets and their characteristics.

Wrong options:

• synonymy: 'Synonymy' refers to the relationship between words with similar meanings, which does not fit the context of a science meeting.
• indecisively: 'Indecisively' is an adverb describing uncertainty, which does not fit the grammatical structure needed here.
• limply: 'Limply' is an adverb meaning lacking firmness or strength, which is irrelevant in this context.

Correct answer: announced

Explanation: The verb 'announced' fits grammatically in the past tense, indicating that researchers made a statement. It aligns contextually with the narrative of sharing findings at a science meeting.

Meaning: 'Announced' means to make something known publicly, indicating that researchers shared their discovery about Comet Hartley 2.

Wrong options:

• lounged: 'Lounged' is a verb meaning to sit or lie in a relaxed way, which does not fit the context of making a formal statement.
• discount: 'Discount' is a verb meaning to disregard or minimize, which does not fit the context of sharing information.
• runabout: 'Runabout' is a noun referring to a small vehicle or a casual term for a short trip, which is irrelevant in this context.