In Psychopathology Research,groups of Schizophrenic Patients Are Sometimes Compared to Groups

In psychopathology research,groups of schizophrenic patients are sometimes compared to groups of "patient controls." In such research,patient controls are used to

A) determine whether certain factors are specific to schizophrenia or apply to other disorders.
B) identify the differences between people with schizophrenia and people without mental disorder.
C) control for differences between subjects in severity of illness.
D) control for subject and experimenter bias.