challenge: In two-dimensional parabolic coordinates, what are the equations used to define the coordinates (σ, τ) in terms of Cartesian coordinates?
  
  A. σ = x^2 + y^2, τ = x^2 - y^2
  B. σ = x^2 + y^2, τ = 2xy
  C. σ = x + y, τ = x - y
  D. σ = x^2 - y^2, τ = 2xy
  Answer: 