Which of the Following Is True About the Immune System

Which of the following is true about the immune system and stress?

A) NK cells are a type of white blood cell that is more likely to be present in high stress circumstances.
B) Higher levels of NK cells in stressful situations indicate a weakened immune system.
C) Immune system functioning increases with normal aging.
D) Aerobic fitness is related to the presence of a lower level of senescent T cells.