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?
【选项】Those in the study who survived into the 1980s had, in 1976, strengthened their immune systems through drug therapy.
选项是否正确？