During a Well-Baby Checkup, the Mother of an Infant States

During a well-baby checkup, the mother of an infant states, "Even if he is occupied with a toy, he cries as soon as he notices I have left the room." Which explanation by the nurse is best?

A) "Your baby does not know you exist if he cannot see you."
B) "Babies learn very quickly how to get an adult's attention."
C) "You should move the baby with you if you leave the room."
D) "Just ignore him; he will soon learn that you are still present."