How Has Research on Comorbidity Changed the Focus of Epidemiological

How has research on comorbidity changed the focus of epidemiological research?

A) shifted the focus from the psychotic disorders to milder disorders that affect more people
B) higher than expected prevalence of mental retardation led to more emphasis on intellectual ability
C) shifted the focus from counting the number of people with a disorder to measuring the functional impairment associated with the problems
D) evidence for the biological etiology of more mental disorders has switched the focus to identifying the genes responsible for particular disorders