Meaning: 'Date' refers to a particular day or time, in this case, indicating the present moment in relation to Sbordoni's discoveries.

Wrong options:

• now: 'Now' indicates the present moment but lacks the specificity of a time reference that 'date' provides.
• present: 'Present' is a noun that can refer to the current time but does not convey the same sense of a specific point in time as 'date.'
• today: 'Today' refers to the current day but is less formal and specific than 'date,' which is more appropriate in this context.

Correct answer: hitherto

Explanation: The term 'hitherto' indicates something that has been the case up to this point in time, which fits perfectly in the context of Sbordoni's discoveries of species that were previously unknown.

Meaning: 'Hitherto' means 'up to this time' or 'until now,' emphasizing that these species were not known before Sbordoni's research.

Wrong options:

• hereby: 'Hereby' means 'by this means' and does not fit the context of indicating a time frame related to discoveries.
• henceforth: 'Henceforth' refers to a time after the present, which contradicts the context of past discoveries.
• herewith: 'Herewith' means 'with this' and does not relate to the timeline of discoveries, making it contextually inappropriate.

Correct answer: alone

Explanation: 'Alone' is used here to indicate that Sbordoni found the species without any other species present, which fits grammatically and contextually in the sentence.

Meaning: 'Alone' means 'without others' or 'solitarily,' emphasizing the singularity of the finding in that chamber.

Wrong options:

• exclusively: 'Exclusively' suggests that something is limited to a particular group, which does not fit the context of simply finding species.
• only: 'Only' implies a limitation but does not convey the same sense of solitude as 'alone' does in this context.
• solely: 'Solely' indicates being the only one but is more formal and does not match the tone of the sentence as well as 'alone.'

Correct answer: distinct

Explanation: 'Distinct' is the most appropriate choice as it conveys that the species are clearly different from one another, which is essential in the context of biodiversity.

Meaning: 'Distinct' means 'clearly different or unique,' highlighting the variety of species found.

Wrong options:

• dissimilar: 'Dissimilar' means not alike but is less commonly used in this context and does not convey the same clarity as 'distinct.'
• disparate: 'Disparate' suggests a significant difference but carries a connotation of inequality that does not fit the context of species diversity.
• disconnected: 'Disconnected' implies a lack of connection, which does not accurately describe the relationship between the species found.

Correct answer: retreated

Explanation: 'Retreated' fits grammatically and contextually as it indicates that the species moved back to underground habitats to avoid extinction, aligning with the narrative of adaptation.

Meaning: 'Retreated' means to withdraw or move back, particularly in the context of species adapting to environmental changes.

Wrong options:

• retired: 'Retired' typically refers to ceasing work or activity, which does not apply to species in this context.
• shrank: 'Shrank' implies a reduction in size rather than a movement or migration, making it contextually inappropriate.
• departed: 'Departed' suggests leaving but does not convey the idea of moving back to a previous state or location as 'retreated' does.

Correct answer: account for

Explanation: 'Account for' is the correct phrase as it means to explain or justify something, which fits the context of explaining the reasons behind the migration of species.

Meaning: 'Account for' means to provide a reason or explanation for something, particularly in relation to the migration discussed.

Wrong options:

• conjure up: 'Conjure up' means to bring to mind or create, which does not fit the context of explaining a phenomenon.
• bear out: 'Bear out' means to confirm or support, which does not align with the need to explain the migration.
• carry off: 'Carry off' implies taking something away, which is unrelated to the context of explaining the migration of species.