What Was the Important Difference Between the Test of Mathematical

What was the important difference between the test of mathematical ability conducted by Mehler and Bever and that conducted by Piaget?

A) Mehler and Bever used candy and thus increased the children's motivation.
B) Piaget expected the children to be able to write their answers, not verbalize them.
C) The children in Piaget's study had no previous math experience.
D) The children in Mehler and Bever's study had previously attended preschool.