I want to know the answer to the following question:
What format has never been implemented on CDs?
All the background I'm provided with is that "Monaural audio has no existing standard on a Red Book CD; thus, mono source material is usually presented as two identical channels in a standard Red Book stereo track (i.e., mirrored mono); an MP3 CD, however, can have audio file formats with mono sound.". Is that enough to answer the question?
Answer: no