Why Do Adults Become More Susceptible to Disease as They

Why do adults become more susceptible to disease as they age?

A) Fewer antibodies are produced and T cells become less efficient.
B) The thymus gland becomes excessively large and interferes with the production of B cells.
C) More B cells and T cells are produced than can be absorbed by the bone marrow and the thymus gland.
D) The thymus gland and the bone marrow produce antibodies that attack normal body cells.