challenge: What was the main conclusion of the Bunker experiment?
  
  A. Humans have an intrinsic clock with a period of exactly 24 hours
  B. The experiment was inconclusive about the existence of an intrinsic clock in humans
  C. Humans do not have an intrinsic circadian clock
  D. Humans have an intrinsic clock with a period greater than 24 hours
  Answer: 