challenge: What is the asymptotic formula for the number of (labeled) planar graphs on n vertices?
  
  A. g * n^(-7/2) * gamma^(2n) * n!
  B. g * n^(-5/2) * gamma^n * n!
  C. g * n^(-7/2) * gamma^n * n!
  D. g * n^(-5/2) * gamma^(2n) * n!
  Answer: 