A two-year study beginning in 1977 found that, among 85-year-old people, those whose immune systems were weakest were twice as likely to die within two years as others in the study. The cause of their deaths, however, was more often heart disease, against which the immune system does not protect, than cancer or infections, which are attacked by the immune system. Which of the following, if true, would offer the best prospects for explaining deaths in which weakness of the immune system, though present, played no causal role? ['Some of the drugs that had been used to treat the symptoms of heart disease had a side effect of weakening the immune system.', 'Those in the study who survived into the 1980s had, in 1976, strengthened their immune systems through drug therapy.', 'Most of those in the study who survived beyond the two-year period had recovered from a serious infection sometime prior to 1978.', 'There were twice as many infections among those in the study with the weakest immune systems as among those with the strongest immune systems.']