  We study the sensitivity of presupernova evolution and supernova
nucleosynthesis yields of massive stars to variations of the helium-burning
reaction rates within the range of their uncertainties. We use the current
solar abundances from Lodders (2009) for the initial stellar composition. We
compute a grid of 12 initial stellar masses and 176 models per stellar mass to
explore the effects of independently varying the 12^C(a,g)16^O and 3a reaction
rates, denoted R_a12 and R_3a, respectively. The production factors of both the
intermediate-mass elements (A=16-40) and the s-only isotopes along the weak
s-process path (70Ge, 76Se, 80Kr, 82Kr, 86Sr, and 87Sr) were found to be in
reasonable agreement with predictions for variations of R_3a and R_a12 of
+/-25%; the s-only isotopes, however, tend to favor higher values of R_3a than
the intermediate-mass isotopes. The experimental uncertainty (one standard
deviation) in R_3a(R_a12) is approximately +/-10%(+/-25%). The results show
that a more accurate measurement of one of these rates would decrease the
uncertainty in the other as inferred from the present calculations. We also
observe sharp changes in production factors and standard deviations for small
changes in the reaction rates, due to differences in the convection structure
of the star. The compactness parameter was used to assess which models would
likely explode as successful supernovae, and hence contribute explosive
nucleosynthesis yields. We also provide the approximate remnant masses for each
model and the carbon mass fractions at the end of core-helium burning as a key
parameter for later evolution stage
