In a study comparing the genetic contribution to eating disorders, researchers found that in ___% of identical twin pairs both twins had bulimia nervosa, while that number was ___% of fraternal twin pairs. Although this does not rule out the influence of environmental and sociocultural factors, it does present compelling evidence for a genetic contribution to these conditions.

a. 47; 31
b. 9;2
c. 68; 22
d. 23; 9