Four-year-old Billy always wants to drink his milk from his own tall, thin glass. His eight-year-old brother seems unconcerned about what kind of cup or glass his milk is served in. Why might Billy be so concerned about the glass? A. According to Piaget, because Billy used the glass before, it is now his "rule" that he use it every time he drinks milk. B. According to Erikson, he is experimenting with his control over his environment; he wants to see if he can control by demanding the same glass at each meal C. According to Piaget, he is experimenting with object permanence; he keeps checking to make certain his glass exists even though he doesn't see it between meals. D. According to Piaget, Billy lacks conservation of volume and thinks that the tall, thin glass holds more milk than the shorter, wider glass.