OG19 verbal CR 156

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.

A. there were twice as many infections among those in the study with the weakest immune systems as among those with the strongest immune systems.
B. the majority of those in the study with the strongest immune system died from infection or cancer by 1987
C. some of the drugs that had been used to treat the symtoms of heart disease had a side effect of weaking the immune system
D most of those in the study who survived beyond the two-year period had recovered from a serious infection sometimes prior to 1978
E those in the study who survived into the 1980s had in 1976 strengthened their immune systems through drug therapy.

没太理解这题的意思，属于什么推理方式呢？请教大牛