Adoption Studies Tend to Study Mothers More Often Than Fathers

Adoption studies tend to study mothers more often than fathers because

A) mothers tend to have a stronger genetic influence on their children than fathers do.
B) mothers tend to have a stronger environmental influence on their children than fathers do.
C) fathers generally have less genetic and environmental influence on their children's development than mothers do.
D) it's harder to get information about the biological fathers than about the mothers.