Conventional wisdom suggests vaccinating elderly people first in flu season, because they are at greatest risk of dying if they contract the virus. This year's flu virus poses particular risk to elderly people and almost none at all to younger people, particularly children. Nevertheless, health professionals are recommending vaccinating children first against the virus rather than elderly people. Which of the following, if true, provides the strongest reason for the health professionals' recommendation? ['Children who catch one strain of the flu virus and then recover are likely to develop immunity to at least some strains with which they have not yet come in contact.', 'Children are particularly unconcerned with hygiene and ttherefore are the group most responsible for spreading the flu virus to others.', "The vaccinations received last year will confer no immunity to this year's flu virus.", 'Children are no more likely than adults to have immunity to a particular flu virus if they have never lived through a previous epidemic of the same virus.']