A new government policy has been developed to avoid many serious cases of influenza. This goal will be accomplished by the annual vaccination of high-risk individuals: everyone 65 and older as well as anyone with a chronic disease that might cause them to experience complications from the influenza virus. Each year' s vaccination will protect only against the strain of the influenza virus deemed most likely to be prevalent that year, so every year it will be necessary for all high-risk individuals to receive a vaccine for a different strain of the virus. Which one of the following is an assumption that would allow the conclusion above to be properly drawn? ['The number of individuals in the high-risk group for influenza will not significantly change from year to year.', 'The likelihood that a serious influenza epidemic will occur varies from year to year.', 'Each year the strain of influenza virus deemed most likely to be prevalent will be one that had not previously been deemed most likely to be prevalent.', 'No vaccine for the influenza virus protects against more than one strain of that virus.']