Several researchers have claimed that Piaget underestimated children because they have shown that children can pass some concrete operational tasks earlier than Piaget thought. How would Piaget respond to them?

a. "Your tests are not culturally relevant to the children you are testing."
b. "Your methods must be flawed because you are teaching them the skills and then testing them immediately."
c. "Kids these days must be smarter than they were when I developed my theory."
d. "It's not enough for children to grasp some aspects of concrete operations; they have to master all of them."

Answer: D