  We employ analytical and numerical chemical evolution models to study
observed trends in abundance ratios involving carbon, nitrogen, and oxygen.
Several sets of published stellar yields for both intermediate-mass and massive
stars are considered, and the most appropriate sets are selected through the
use of analytical models. These yields are then used in the numerical models to
match observed data trends in C/O, N/O, and O/H. We conclude that the principal
production site for carbon is massive stars, while that for nitrogen is
intermediate-mass stars.
