What Olshansky and his team member discovered shows that
A．it is possible for the average life span to increase to 120 years, if death rates are reduced.
B．the average human life span will increase a little, if the death rates are reduced.
C．the upper limits of life bears no relation with the change of death rates,
D．the average human life span will increase a lot, if no one died before reaching age 50.