According to Piaget, How Can We Determine Whether a Child

According to Piaget, how can we determine whether a child has the concept of object permanence?

A) Watch the child's eye movements when the object moves.
B) Place a toy behind a barrier and see whether the child retrieves it.
C) Squash some clay or pour liquid into a different container and see whether the child thinks it still is the same amount.
D) See whether the child consistently prefers one toy to another.