challenge: What is the primary function of a photomultiplier tube (PMT) in a scintillation detector?
  
  A. To detect incoming photons by exciting charge carriers directly in the silicon
  B. To absorb the light emitted by the scintillator and re-emit it in the form of electrons
  C. To amplify the electrical pulse generated by the scintillator
  D. To operate in avalanche mode and enable each pixel to be sensitive to single photons
  Answer: 