Meaning: 'The result of' means something that happens because of a particular cause, indicating that the virus was caused by a genetic mutation.

Wrong options:

• instead of: 'Instead of' suggests an alternative, which does not fit the context of explaining causation here.
• in spite of: 'In spite of' indicates a contrast, which is not appropriate for describing the cause of the virus.
• as well as: 'As well as' is used to add information, but does not convey the causal relationship needed in this context.

Correct answer: over

Explanation: The preposition 'over' is used to indicate movement across a surface or area, which fits the context of the virus spreading across vast distances, aligning with the grammatical structure of the sentence.

Meaning: 'Over' in this context means across or throughout, indicating the extensive reach of the virus.

Wrong options:

• to: 'To' indicates direction but does not convey the sense of spreading across distances as effectively as 'over'.
• in: 'In' suggests being inside something, which does not fit the context of spreading across distances.
• of: 'Of' indicates belonging or association, which does not apply to the action of spreading.

Correct answer: rapidity

Explanation: The noun 'rapidity' fits grammatically as it serves as the subject complement, describing the manner in which the virus spread, and it aligns with the formal tone of the text.

Meaning: 'Rapidity' means the quality of being rapid or fast, emphasizing the quick nature of the virus's spread.

Wrong options:

• speedily: 'Speedily' is an adverb and does not fit grammatically as a noun in this context.
• quickened: 'Quickened' is a verb form that does not serve as a noun, making it unsuitable here.
• rapidly: 'Rapidly' is also an adverb, which does not fit the grammatical structure required.

Correct answer: within

Explanation: The preposition 'within' indicates a time frame, fitting grammatically and contextually as it describes the time frame in which death occurs after infection.

Meaning: 'Within' means inside a certain period of time, indicating that death happens during that time frame after infection.

Wrong options:

• into: 'Into' suggests movement towards something, which does not fit the context of time here.
• from: 'From' indicates a starting point, which does not convey the time frame needed in this context.
• at: 'At' indicates a specific point in time, which does not capture the duration implied by 'within'.

Correct answer: responsible

Explanation: The adjective 'responsible' appropriately indicates causation, fitting grammatically and contextually as it describes the role of the Spanish flu in causing deaths, aligning with the overall message of the sentence.

Meaning: 'Responsible' means being the cause of something, indicating that the Spanish flu was the cause of many deaths.

Wrong options:

• guilty: 'Guilty' implies blame or wrongdoing, which does not fit the context of discussing a virus's impact.
• able: 'Able' indicates capability, which does not relate to the causative context of the sentence.
• plausible: 'Plausible' means reasonable or believable, but does not convey the direct causation needed in this context.