challenge: What is a necessary condition for a triangle to be called the tangential triangle of a reference triangle?
  
  A. The tangential triangle must be a right triangle
  B. The tangential triangle's angles must be equal to the reference triangle's angles
  C. The tangential triangle's sides must be equal in length to the reference triangle's sides
  D. The tangencies of the tangential triangle with the circle must also be the vertices of the reference triangle
  Answer: 