challenge: What is the criterion for a subset X of real numbers to be considered open in the compact complement topology?
  
  A. If its complement in the standard Euclidean topology is connected
  B. If its complement in the standard Euclidean topology is dense
  C. If its complement in the standard Euclidean topology is countable
  D. If its complement in the standard Euclidean topology is compact
  Answer: 