African immigrants living in London neighborhoods were more likely to be diagnosed with ...

African immigrants living in London neighborhoods were more likely to be diagnosed with schizophrenia if they lived in a predominantly White neighborhood than if they lived in a primarily non-White neighborhood (Boydell et al., 2001). According to the text, what do these findings suggest?
 
  a. Persons susceptible to schizophrenia experienced downward drift to these neighborhoods.
  b. Certain ethnicities are more biologically vulnerable to schizophrenia than others.
  c. The stress of social isolation due to minority status might contribute to the risk of schizophrenia.
  d. Persons may self-select neighborhoods in order to live among those similar to themselves.