challenge: What is the current state of knowledge about the premiere of Mozart's Symphony No. 39?
  
  A. It is unclear, and it is uncertain whether the symphony was ever performed in Mozart's lifetime
  B. It is unknown, but likely occurred in a series of 'Concerts in the Casino'
  C. It is well-documented and occurred in 1787
  D. It is established that the symphony was never performed in Mozart's lifetime
  Answer: 