The Number of Stem Cells Appears to Gradually Decline with Age;

The number of stem cells appears to gradually decline with age; if so, how might this decline be explained?

A) To maintain a stable number of stem cells, one daughter cell of each division would need to revert back to a stem cell; the rate of reversion must decline with age.
B) Lacking specialization, stem cells must become more susceptible to disease processes than normal cells; eventually this susceptibility reduces their number.
C) As they age, stem cells evidently lose their vitality, quit dividing, and die of old age without leaving any additional daughter cells.
D) Continuous division with progressively shortened telomeres eventually produces a stem cell with critically missing DNA and a loss of an ability to further divide.