Two Photons Are Emitted as an Electron De-Excites from the Third

Multiple Choice

Two photons are emitted as an electron de-excites from the third quantum level to the second, and then to the ground state. The sum of the frequencies of the two photons equals the frequency of the single photon that would be emitted if the de-excitation were from the third to the ground state directly. This is according to

A) the physicist W. Ritz.
B) energy conservation.
C) both of these
D) neither of these