Why was Charles Darwin initially puzzled by the observation that worker bees die when they sting an ...

Why was Charles Darwin initially puzzled by the observation that worker bees die when they sting an intrud-er to their hive? (The bee's stinger is ripped from the bee's abdomen, causing a fatal injury.)
 
  a. All defensive behaviors result in the guaranteed destruction of the defender.
  b. Such self-sacrifice is widespread in many different groups of animals.
  c. He found it hard to imagine how such behavior could evolve, as a defending worker's chance of survival is zero.
  d. The stinging apparatus appeared to be too complex to have evolved.