challenge: What led Michael Sigismund Frank to attempt to recover the lost secret of painting in glass?
  
  A. A transaction between his friend Wirth and an Englishman, in which the Englishman paid a high price for fragments of ancient coloured glass
  B. A visit to a museum where he saw examples of medieval stained glass
  C. A commission from a wealthy patron to create a stained glass window
  D. A discovery of ancient glass fragments in his own shop
  Answer: 