The large $SU(3)_F$ violations in the amplitudes for the $D$ decays
into a pair of pseudoscalar mesons are mainly accounted by the final
state interaction and by contributions related to the non conservation
of the strangeness changing vector currents of the octet.
An approximate selection rule for the contributions proportional to
$V_{cb} V^\ast_{ub}$ allows to predict the CP violating asymmetries
in terms of only one parameter depending on the penguin contribution.
The enhancement of the octet final states with respect to the $27$,
larger than expected, may be related to the presence of the nonet of
scalar mesons with masses around the one of the $D$'s. A similar
mechanism may be responsible for the dominance of the
$\Delta I = \frac{1}{2}$ term in $K$ decays related to the large
$f^0$ resonance with a mass near to one of the decaying particles.