  For hadronic two-body decays of charmed mesons involving $\eta$ or $\eta'$,
resonance-induced final-state interactions (FSI) that mimic the W-exchange or
the W-annihilation topology can play an essential role. In particular, the
decays $D^0\to\bar K^0\eta'$ and $D^+\to\pi^+\eta$, which are largely
suppressed in the absence of FSI, are enhanced dramatically by resonant FSI. It
is stressed that the effect of resonant FSI is negligible for
$\rho^+(\eta,\eta')$ final states because of the mismatch of the G parity of
$\rho^+(\eta,\eta')$ and the J=0, I=1 meson resonance. We argue that a possible
gluon-mediated process in which two gluons couple directly to the gluonic
component of the $\eta'$, e.g. the gluonium, rather than to the flavor-singlet
$\eta_0$, can enhance both modes $D_s^+\to\rho^+\eta'$ and
$D_s^+\to\rho^+\eta$, especially the former; that is, this new mechanism can
account for the unexpectedly large branching ratio of $\rho^+\eta'$ without
suppressing $\rho^+\eta$.
