For Piaget, Cognitive Advances at Each Stage Are Reflected Not

For Piaget, cognitive advances at each stage are reflected not just in the answers children devise for problems, but in what other critical and demonstrable way?

A) in their explanations and supporting defense of how they arrived at the solutions
B) in the emotional maturity with which they solve the problems
C) in how long they stay on task
D) in the advancement of their memory abilities