What Is the Most Direct Evidence for a Genetic Explanation

What is the most direct evidence for a genetic explanation for substance use disorders?

A) Surveys of adults with alcohol use disorder
B) Higher rates of alcohol preference demonstrated in breeding studies
C) The study finding that fraternal twins have a concordance rate of 30 percent for substance use disorder
D) The study finding that individuals with substance use disorder are more likely to have an abnormal D2 receptor gene