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.