As we age and enter older adulthood, which of the following changes is LEAST likely to occur?

a. Our immune function becomes stronger, because we have been exposed to so many diseases that immunities have developed.
b. Respiratory infections become more common because aging involves a lower capacity of oxygen intake.
c. The reserve capacity of our cardiovascular system becomes smaller.
d. Our sense of balance is compromised, so we are more likely to fall and hurt ourselves.

a