What is the general conclusion of researchers who have investigated the influence of genetic factors on attention-deficit/hyperactivity disorder?

a. Genetic factors strongly contribute to this disorder.
b. The concordance rate for MZ twins is high, but symptoms appear to be reactions to other mental disorders.
c. Genetic factors seem to play no role in the development of this disorder.
d. The concordance rate among dizygotic twins is higher than it is for monozygotic twins.

Answer: a.