challenge: What is believed to be the main source used by the Didriks Krönika, a late medieval Old Swedish adaptation of the Þiðreks saga?
  
  A. A German manuscript of the Þiðreks saga
  B. The oldest extant manuscript of the Þiðreks saga (Mb)
  C. An oral tradition of the Þiðreks saga
  D. A Danish manuscript of the Þiðreks saga
  Answer: 