It Is Difficult to Separate Environmental and Genetic Influences on IQ

It is difficult to separate environmental and genetic influences on IQ even in studies of adopted children because

A) although the separation of mother and infant does not always occur at birth, the biological mother provides none of the postbirth environment.
B) the adoptive mother makes important prenatal environmental contributions to intelligence.
C) adoption agencies often try to match characteristics of adoptive parents with those of biological parents.
D) All of the alternatives are correct.