Meaning: 'Acting' means performing a function or role, indicating that ozone serves as a protective barrier against UV light.

Wrong options:

• Imagining: Imagining suggests a mental process rather than a physical action, making it contextually inappropriate here.
• Moving: Moving implies physical relocation, which does not accurately describe the role of ozone in absorbing UV light.
• Rotating: Rotating indicates a circular movement, which is not relevant to the protective function of ozone.

Correct answer: linger

Explanation: The word 'linger' is appropriate as it suggests that the by-products remain in the atmosphere for an extended period, fitting grammatically and contextually with the sentence's meaning about the persistence of these chemicals.

Meaning: 'Linger' means to stay in a place longer than necessary, indicating that these chemicals do not disappear quickly from the atmosphere.

Wrong options:

• disappear: Disappear implies a complete absence, which contradicts the context of the chemicals affecting the ozone layer.
• move: Move suggests a change in location, which does not convey the idea of the chemicals remaining in the atmosphere.
• dispose: Dispose means to get rid of something, which does not fit the context of the chemicals' presence in the atmosphere.

Correct answer: expect

Explanation: The word 'expect' is suitable as it indicates a prediction or belief about the future, fitting grammatically and contextually with the sentence regarding experts' views on the ozone hole's closure.

Meaning: 'Expect' means to anticipate or look forward to something happening, in this case, the closure of the ozone hole by 2050.

Wrong options:

• know: Know implies certainty rather than prediction, which does not align with the context of future expectations.
• said: Said is a past tense verb that does not fit the future-oriented context of the sentence.
• found: Found suggests discovery or realization, which does not convey the predictive aspect needed in this context.

Correct answer: counterpart

Explanation: The word 'counterpart' is correct as it refers to something that has a similar function or role, fitting grammatically and contextually with the comparison to last year's ozone hole.

Meaning: 'Counterpart' means something that corresponds to or complements another, indicating a comparison with the previous year's hole.

Wrong options:

• competitor: Competitor implies rivalry, which does not fit the context of comparing the size of the ozone holes.
• traitor: Traitor suggests betrayal, which is irrelevant and inappropriate in this context.
• sibling: Sibling implies a familial relationship, which does not accurately describe the relationship between the two ozone holes.