Which of the Following Is a Likely Reason for the Discrepancy

Which of the following is a likely reason for the discrepancy between Piaget and later researchers' (especially Baillargeon') findings about the age at which infant/toddlers attain object permanence?

A) Piaget's work was conducted more than half a century ago, so it is no longer relevant.
B) Piaget used more demanding reaching measures than his more contemporary colleagues.
C) Baillargeon and his colleagues did not use looking time in their tests.
D) Piaget did not believe that object-retrieval tasks were based on infants'/toddlers' conscious knowledge.