Meaning: 'Over' in this context means 'across' or 'throughout,' suggesting the virus spread widely across different regions.

Wrong options:

• to: The preposition 'to' indicates direction towards a specific point, which does not convey the idea of widespread movement as effectively as 'over.'
• in: The preposition 'in' suggests being inside or within a space, which does not fit the context of spreading across distances.
• of: The preposition 'of' indicates belonging or association, which does not apply to the action of spreading.

Correct answer: rapidity

Explanation: The noun 'rapidity' refers to the quality of being rapid or fast, fitting grammatically as it serves as the subject complement describing the manner of the virus's spread. It aligns contextually with the idea of speed in the sentence.

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

Wrong options:

• speedily: The adverb 'speedily' describes how something is done but does not fit grammatically as it requires a noun form in this context.
• quickened: The past participle 'quickened' implies something has been made faster, which does not fit the grammatical structure needed here.
• rapid: The adjective 'rapid' describes something fast but does not function as a noun, making it unsuitable for this blank.

Correct answer: within

Explanation: The preposition 'within' indicates a time frame or limit, fitting grammatically as it specifies the time frame in which death occurs after infection. It is contextually appropriate as it conveys the urgency of the situation.

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

Wrong options:

• into: The preposition 'into' suggests movement towards the inside of something, which does not fit the context of time as needed here.
• at: The preposition 'at' indicates a specific point in time, which does not convey the duration implied by 'within.'
• from: The preposition 'from' indicates the starting point of a time period, which does not align with the context of when death occurs.

Correct answer: responsible

Explanation: The adjective 'responsible' indicates being the cause of something, fitting grammatically as it describes the subject (the Spanish flu) in relation to the deaths. It is contextually appropriate as it highlights the flu's role in the high death toll.

Meaning: 'Responsible' means 'having a duty to take care of something or being the cause of something,' indicating that the Spanish flu caused many deaths.

Wrong options:

• guilty: The adjective 'guilty' implies wrongdoing or blame, which does not fit the context of a virus causing deaths without moral implications.
• able: The adjective 'able' indicates capability, which does not relate to the cause of deaths in this context.
• plausible: The adjective 'plausible' means something that seems reasonable or probable, which does not convey the direct causative relationship needed here.