challenge: What is the estimated age of the Charity Shoal Crater?
  
  A. About 460 million years ago, during the Middle Ordovician period
  B. About 100 million years ago, during the Cretaceous period
  C. About 200 million years ago, during the Triassic period
  D. About 650 million years ago, during the Neoproterozoic era
  Answer: 