Meaning: 'Verge' refers to the edge or border of something, suggesting that the species are close to extinction.

Wrong options:

• completely: 'Completely' is an adverb that indicates totality, which does not fit the context of being on the brink of extinction.
• quickly: 'Quickly' is an adverb that describes speed, which is irrelevant to the context of extinction.
• natural: 'Natural' is an adjective that describes something occurring in nature, which does not fit the context of being on the edge of extinction.
• diffusion: 'Diffusion' refers to the process of spreading something more widely, which does not relate to the context of species becoming extinct.

Correct answer: natural

Explanation: The term 'natural' is appropriate as it describes the original environment where species thrive. It fits grammatically and contextually, emphasizing the importance of preserving their natural habitats.

Meaning: 'Natural' refers to something that exists in or is caused by nature, indicating the original habitat of the animals.

Wrong options:

• completely: 'Completely' is an adverb that does not describe a type of habitat and is irrelevant in this context.
• quickly: 'Quickly' is an adverb that describes speed, which does not apply to the concept of habitat.
• verge: 'Verge' refers to the edge of something, which does not describe a habitat and is not contextually appropriate.
• diffusion: 'Diffusion' refers to spreading, which does not relate to the concept of a habitat.

Correct answer: obliged

Explanation: The word 'obliged' fits grammatically as it indicates a moral responsibility to act. It aligns with the context of intervening for the sake of endangered species, emphasizing ethical considerations.

Meaning: 'Obliged' means having a moral duty to do something, in this case, to intervene for the species' survival.

Wrong options:

• completely: 'Completely' is an adverb that does not convey a sense of obligation and is irrelevant in this context.
• quickly: 'Quickly' is an adverb that describes speed, which does not relate to moral responsibility.
• natural: 'Natural' is an adjective that does not convey a sense of obligation and is not contextually appropriate.
• verge: 'Verge' refers to the edge of something, which does not fit the context of moral duty.

Correct answer: opportunity

Explanation: The term 'opportunity' fits grammatically as it indicates a chance to do something. It aligns contextually with the idea of studying animals, emphasizing the benefit zoos provide to scientific research.

Meaning: 'Opportunity' refers to a favorable chance or situation to do something, in this case, to study animals.

Wrong options:

• completely: 'Completely' is an adverb that does not convey a chance or situation and is irrelevant in this context.
• quickly: 'Quickly' is an adverb that describes speed, which does not relate to the concept of studying animals.
• natural: 'Natural' is an adjective that does not convey a chance or opportunity and is not contextually appropriate.
• verge: 'Verge' refers to the edge of something, which does not fit the context of providing a chance.

Correct answer: virtually

Explanation: The word 'virtually' fits grammatically as it serves as an adverb modifying 'impossible.' It aligns contextually by emphasizing that studying these animals in the wild is nearly impossible, highlighting the unique role of zoos.

Meaning: 'Virtually' means nearly or almost, indicating that studying animals in the wild is not feasible.

Wrong options:

• completely: 'Completely' is an adverb that indicates totality, which does not fit the context of something being nearly impossible.
• quickly: 'Quickly' is an adverb that describes speed, which does not relate to the concept of feasibility.
• natural: 'Natural' is an adjective that does not convey the idea of possibility and is not contextually appropriate.
• verge: 'Verge' refers to the edge of something, which does not fit the context of describing the feasibility of studying animals.