challenge: What ranking list was used as the basis for seedings in the 2021 Players Championship?
  
  A. Two-year ranking list up to the 2020 World Snooker Championship
  B. Three-year ranking list up to the 2020 UK Championship
  C. Five-year ranking list up to the 2019 Masters
  D. One-year ranking list up to the 2021 Welsh Open
  Answer: 