Based on the Research into Genetic Factors,what Can Be Concluded

Based on the research into genetic factors,what can be concluded about the risk of having schizophrenia?

A) The risk is unrelated to the presence of schizophrenia in family members.
B) The risk is greatest among individuals when a fraternal twin has suffered from the disorder.
C) The risk is greatest among individuals when both parents have suffered from the disorder.
D) The risk is greatest when the individual has been exposed to toxic substances in utero.