A Child Comes Down with the Chickenpox, Caused by the Varicella-Zoster

A child comes down with the chickenpox, caused by the varicella-zoster virus.Her mother takes care of the child, exposing herself to the virus, but does not come down with chickenpox herself.What is the most likely explanation for this?

A) The disease is not very contagious.
B) The symptoms are less severe in adults, so the mother developed only a mild case that went unnoticed.
C) The mother harbored memory cells against the virus, having had chickenpox when she was young.
D) The mother carried antigens against the virus, having been vaccinated earlier.
E) The mother's immune system was strong enough to prevent her coming down with the disease.