Meaning: 'Few' refers to a small number of something, in this case, indicating that the number of years is not large.

Wrong options:

• for: 'For' is a preposition that indicates a duration of time but does not quantify the number of years, making it grammatically incorrect in this context.
• more: 'More' suggests a greater quantity but does not specify a number, which does not fit the requirement for a quantifier in this context.
• much: 'Much' is typically used with uncountable nouns and does not work with the plural noun 'years,' making it grammatically incorrect.

Correct answer: against

Explanation: The word 'against' is used to describe the action of icebergs colliding or interacting with each other, fitting grammatically as a preposition that indicates the relationship between the icebergs. Contextually, it conveys the physical interaction that is being studied.

Meaning: 'Against' means in opposition to or in contact with, indicating how icebergs interact with one another.

Wrong options:

• within: 'Within' suggests being inside something, which does not accurately describe the external interaction of icebergs with each other.
• about: 'About' indicates a topic or subject but does not convey the physical action of icebergs colliding, making it contextually inappropriate.
• through: 'Through' implies movement from one side to another, which does not fit the context of icebergs grinding against each other.

Correct answer: which

Explanation: The word 'which' is a relative pronoun that introduces a clause providing additional information about the sounds, fitting grammatically as it connects the clause to the noun 'extent.' Contextually, it specifies the sounds being discussed and their contribution to ocean noise.

Meaning: 'Which' is used to refer to a specific thing or things from a defined set, in this case, the sounds of icebergs.

Wrong options:

• why: 'Why' is an interrogative adverb that asks for reasons, which does not fit grammatically or contextually in this sentence.
• what: 'What' is also an interrogative pronoun that does not serve the purpose of linking the clause to the noun 'extent' as required here.
• whether: 'Whether' introduces alternatives or choices, which does not align with the context of discussing the contribution of sounds.

Correct answer: up

Explanation: The word 'up' is used to indicate an increase in temperature, fitting grammatically as an adverb that modifies the verb 'warm.' Contextually, it conveys the idea of rising temperatures at the poles, which is relevant to the discussion of climate change.

Meaning: 'Up' indicates a direction or increase, in this case, referring to the warming of the poles.

Wrong options:

• away: 'Away' suggests a distance from a place, which does not fit the context of temperature change at the poles.
• out: 'Out' implies movement from inside to outside, which does not accurately describe the warming process being discussed.
• off: 'Off' indicates separation or disconnection, which does not relate to the concept of warming in this context.