challenge: What is a characteristic of the refractive index profile of a step-index optical fiber?
  
  A. A uniform refractive index within the core and a sharp decrease at the core-cladding interface
  B. A higher refractive index in the cladding than in the core
  C. A uniform refractive index throughout the fiber
  D. A gradual decrease in refractive index from the core to the cladding
  Answer: 