Genetic analysis of longevity in several species has shown that observed longevity in natural ...

Genetic analysis of longevity in several species has shown that observed longevity in natural populations does not represent the maximum possible longevity that could have been attained via natural selection. What are the implications of this finding?
 
  a. Traits that are responsible for longevity are not under selection.
  b. There are constraints on the ability of selection to stop the breakdown of an organism's body.
  c. Fitness is maximized at an optimal life span that is lower than the maximum life span.
  d. Late-acting mutations are rapidly purged from populations.