challenge: What was the primary modification made to the WAP-4 locomotive to create the WAP-6 variant?
  
  A. Upgrading the braking system to a more advanced model
  B. Increasing the number of passenger cars
  C. Replacing the electric motors with diesel engines
  D. Replacing Co-Co Flexicoil Mark 1 cast bogies with Fabricated Flexicoil Mark IV bogies
  Answer: 