challenge: What was the proposed modification to the Saturn V launch vehicle in the Saturn V-Centaur booster?
  
  A. Replacing the first stage with a solid rocket booster
  B. Reconfiguring the third stage to use a different type of fuel
  C. Adding a fifth stage with a nuclear-powered engine
  D. Replacing the Apollo spacecraft with a Centaur upper stage as a high-energy fourth stage
  Answer: 