Piaget May Have Underestimated Young Children's Cognitive Competence in Causal

Piaget may have underestimated young children's cognitive competence in causal reasoning because:

A) his problems deliberately tried to confuse young children.
B) he relied on verbally presented problems and verbal justifications of reasoning.
C) he used only interviews with his own children as the basis of his conclusions.
D) he required children to manipulate objects they were not yet capable of manipulating.