The state of a quantum mechanical system is described by a wave function. Consider two physical observables that have discrete eigenvalues: observable A with eigenvalues {a}, and observable B with eigenvalues {B}. Under what circumstances can all wave functions be expanded in a set of basis states, each of which is a simultaneous eigenfunction of both A and B ? (a) Only if the values {a} and {3} are nondegenerate (b) Only if A and B commute (c) Only if A commutes with the Hamitonian of the system (d) Only if B commutes with the Hamiltonian of the system