Jean Piaget's Comparative Studies Have Been Criticized Because of Some

Jean Piaget's comparative studies have been criticized because of some controversy related to the age of his subjects. What specifically did this criticism refer to?

A) There were too many children older than 15.
B) There were too many children younger than three.
C) Accurate birth dates were not always available.
D) Children between five and seven were selected from elite schools; others were not studied.
E) Experimenters were often younger than the studied children.