Studies of the Offspring of Nonschizophrenic Co-Twins from Discordant Twin

Studies of the offspring of nonschizophrenic co-twins from discordant twin pairs suggest that

A) genetic factors cause schizophrenia, while environmental factors are essentially unimportant.
B) the heritability of schizophrenia involves the transmission of a single dominant gene.
C) a genetic predisposition to schizophrenia may remain unexpressed in some individuals unless it is released by some unknown environmental factors.
D) environmental factors play a more important role than genetic factors in the origin of schizophrenia.