What Does the Observation That Agreement Rates for Schizophrenia Are

What does the observation that agreement rates for schizophrenia are high in identical twins suggest?

A) That intense emotional bonds trigger schizophrenia.
B) That schizophrenia does not really run in families because twins are so rare.
C) That schizophrenia is imitated in close relationships.
D) That there is a biological basis for the disorder.