challenge: What is the Overhauser effect?
  
  A. The perturbation of nuclear spin level populations when electron spin transitions are saturated by microwave irradiation
  B. A phenomenon where nuclear spin levels are perturbed by thermal fluctuations
  C. A process where nuclear spin transitions are saturated by optical irradiation
  D. A technique used to measure the static magnetic field in metals and free radicals
  Answer: 