challenge: In the context of the Carathéodory–Jacobi–Lie theorem, what is a requirement for the existence of a symplectic chart around a point?
  
  A. The Hamiltonian function must be zero
  B. The Hamiltonian function must be constant
  C. The symplectic form must be zero
  D. The symplectic form must be non-degenerate
  Answer: 