In Comparing the Incidence of Homosexuality-Either Male or Female-Among Pairs

In comparing the incidence of homosexuality-either male or female-among pairs of identical twins, fraternal twins, and adoptive siblings, the research seems to indicate that:

A) genetic factors play a role in determining sexual orientation.
B) genetic factors play either a very small or nonexistent role in sexual orientation.
C) having a very dominant mother or a very permissive father seems to contribute to the development of a homosexual orientation.
D) the closer the degree of genetic relationship, the less likely it is that when one sibling was homosexual, the other would also be homosexual.