For the First Nine Months After Birth,Ronny Knowse Had Not

For the first nine months after birth,Ronny Knowse had not experienced any problems with sickness.After that,however,he had many severe bacterial infections.The reason for the delayed onset of this problem was

A) it took nine months for the thymus to develop fully.
B) he probably was not exposed to any antigens during the first nine months.
C) for the first nine months he had passive natural immunity from antibodies passed to him through the placenta.
D) he had active artificial immunity during the first nine months.
E) he had degeneration of his normal immune function after birth.