The Fact That the Rate of Homosexual Concordance Is Substantially

The fact that the rate of homosexual concordance is substantially higher for identical twins than for nonidentical twins argues in favor of a genetic contribution to sexual orientation.Why does this finding also show us that genetic factors aren't the sole determinants of sexual determinants?

A) If the genetic factors were acting in isolation, then the rates of concordance should be higher for nonidentical twins than for identical twins.
B) If the genetic factors were acting in isolation, then the rates of concordance should include adopted siblings as well as cotwins.
C) If genetic factors were the only determinants involved, then the rate of concordance would be 50 percent.
D) If genetic factors were the only determinants involved, then the rate of concordance would be 100 percent.