Meaning: 'Varying' means changing or differing in size or amount, indicating that sinkholes can have different widths and depths.

Wrong options:

• moving: 'Moving' implies motion, which does not apply to the static nature of sinkholes' dimensions.
• sinking: 'Sinking' suggests a downward movement, which does not accurately describe the widths and depths of sinkholes.
• changing: 'Changing' implies an active process of alteration, which is less precise than 'varying' in this context.

Correct answer: has been dissolved

Explanation: The phrase 'has been dissolved' is in the present perfect tense, indicating a completed action relevant to the present, which is appropriate here as it describes the current state of the underlying rock. This fits the context of sinkholes forming due to past processes.

Meaning: 'Has been dissolved' means that the rock has undergone a process of dissolution, making it susceptible to erosion by acidic water.

Wrong options:

• is dissolving: 'Is dissolving' suggests an ongoing process, which does not fit as the rock must be in a state of dissolution for sinkholes to occur.
• dissolves: 'Dissolves' is in the simple present tense, which does not convey the completed action necessary for the context.
• will be dissolved: 'Will be dissolved' indicates a future action, which is not applicable since the dissolution must have already occurred.

Correct answer: when

Explanation: The word 'when' introduces a conditional clause, indicating the specific time or condition under which water becomes acidic. This fits logically and contextually into the sentence, explaining the process of acid formation.

Meaning: 'When' refers to the time or condition that triggers the acidity of water, specifically in relation to carbon dioxide absorption.

Wrong options:

• why: 'Why' is a question word that does not fit the context of explaining a condition or time.
• then: 'Then' implies a sequence of events, which does not accurately convey the condition under which water becomes acidic.
• before: 'Before' suggests a prior condition, which does not align with the causal relationship being described.

Correct answer: collapses

Explanation: The word 'collapses' accurately describes the action of the earth covering the cavity falling into the hole, fitting grammatically and contextually as it indicates a sudden failure of support.

Meaning: 'Collapses' means to fall down or give way suddenly, which is what happens to the earth covering the cavity when it can no longer support weight.

Wrong options:

• reduces: 'Reduces' implies a decrease in size or amount, which does not accurately describe the action of falling into a hole.
• climbs: 'Climbs' suggests upward movement, which is contrary to the action described in the sentence.
• breaks: 'Breaks' implies a fracture or shattering, which does not convey the same sudden falling action as 'collapses'.

Correct answer: its

Explanation: The pronoun 'its' correctly refers back to 'the earth covering the cavity,' maintaining singular agreement in number and gender, which is essential for clarity in the sentence.

Meaning: 'Its' is a possessive pronoun indicating ownership, referring to the weight that the earth covering must support.

Wrong options:

• her: 'Her' is a feminine pronoun that does not match the neutral reference of 'the earth.'
• his: 'His' is a masculine pronoun that also does not fit the neutral context of the subject.
• their: 'Their' is a plural pronoun that does not agree with the singular subject 'the earth covering.'