  The longitudinal and transverse electromagnetic response functions of
$^{12}$C are computed in a "first-principles" Green's function Monte Carlo
calculation, based on realistic two- and three-nucleon interactions and
associated one- and two-body currents. We find excellent agreement between
theory and experiment and, in particular, no evidence for the quenching of
measured versus calculated longitudinal response. This is further corroborated
by a re-analysis of the Coulomb sum rule, in which the contributions from the
low-lying $J^\pi\,$=$\, 2^+$, $0^+_2$ (Hoyle), and $4^+$ states in $^{12}$C are
accounted for explicitly in evaluating the total inelastic strength.
