Lilly and Billy are sharing graham crackers; there are three total squares left, so Lilly splits one square in half and gives it to Billy. Both children have a total of two crackers, but Billy's crackers are smaller in size than Lilly's. Neither of them notices anything wrong with their sharing. Which Piaget stage have they not mastered yet?

a.

Preoperational

b.

Concrete operations

c.

Primary circular reaction

d.

Secondary circular reaction