Meaning: 'Occupy' means to take up space or reside in a particular area.

Wrong options:

• opposing: 'Opposing' is a present participle that means to resist or fight against, which does not fit the context of describing how much land deserts take up.
• reveal: 'Reveal' is a verb that means to make something known or visible, which does not relate to the concept of land occupation.
• calibrated: 'Calibrated' is a past participle that refers to adjusting or measuring something, which does not apply to the context of land occupation.

Correct answer: increasing

Explanation: The word 'increasing' is a present participle that describes the ongoing action of the expansion of deserts. It fits grammatically as it aligns with the passive construction of the sentence, indicating that the deserts are being affected by this increase.

Meaning: 'Increasing' means becoming greater in size, amount, or degree.

Wrong options:

• threatened: 'Threatened' is a past participle that indicates a state of being in danger, which does not fit the context of describing the action of expansion.
• calibrated: 'Calibrated' does not fit as it refers to measuring or adjusting, which is unrelated to the growth of deserts.
• previously: 'Previously' is an adverb that refers to something that happened before, which does not describe an ongoing action.

Correct answer: previously

Explanation: The adverb 'previously' indicates a time reference, meaning before now. It fits grammatically as it modifies the verb 'exist' to clarify that desert-like conditions did not occur in those areas before the current situation.

Meaning: 'Previously' means at a time before the present.

Wrong options:

• occupy: 'Occupy' is a verb that does not fit here as it does not relate to the timing of existence.
• increasing: 'Increasing' is a present participle that describes an action, which does not fit the context of a past state of existence.
• threatened: 'Threatened' is a past participle that indicates a state of danger, which does not relate to the timing of existence.

Correct answer: threatened

Explanation: The word 'threatened' is a past participle that describes the state of being in danger due to desertification. It fits grammatically as it aligns with the passive construction of the sentence, indicating that the land surface is at risk.

Meaning: 'Threatened' means being in danger of harm or loss.

Wrong options:

• occupy: 'Occupy' does not fit as it refers to taking up space, which is not relevant to the state of being in danger.
• increasing: 'Increasing' describes an action rather than a state, which does not fit the context of being at risk.
• previously: 'Previously' is an adverb that refers to a past time, which does not describe a current state of danger.