What Does Research with Identical Twins on the Role of Heredity

What does research with identical twins on the role of heredity in mental illness indicate?

A) If one twin is diagnosed as mentally ill, it is inevitable that the second twin will eventually develop the same mental illness.
B) There is no greater likelihood of the second twin showing mental illness than there would be in any two nontwin siblings.
C) The second twin has an increased risk of developing the same disorder, but identical twins are usually discordant with respect to mental illness and behavioural disorders.
D) Mental illness shows almost no evidence of heritability.