Dr. Jean Piaget (1952) showed 5-year-old children two identical glass beakers each containing the same amount of liquid. The children had no difficultly understanding that the amount of liquid was the same in both containers. However, when the liquid from one beaker was poured into a tall, thin beaker, the children mistakenly believed that the taller glass had more liquid. Dr. Piaget concluded that children at this age lack the ability to understand that volume remains the same even if its appearance changes. Dr. Piaget was most likely a(n):

A. Developmental psychologist

B. Educational psychologist

C. Social psychologist

D. Child psychologist