Spherical polar coordinates are used to solve the Schrödinger equation for a hydrogen atom because A: the Laplacian operator has its simplest form in spherical polar coordinates. B: Cartesian coordinates would give particle-in-a-box wavefunctions. C: the Schrödinger equation is then separable into 3 ordinary differential equations. D: otherwise the atomic orbitals would violate the Pauli exclusion principle.