Simple stellar population model spectra at solar abundance and three ages, 0.5, 1.0, and 2.0 Gyr, each age labeled on the plot. Carbon stars are included (solid lines) or M stars are substituted for C stars (dashed lines). While CO absorption stays about the same for both cases, there is marked and opposed variation in the H₂O and TiO features of M stars compared to C₂ and CN features due to molecular balancing.