  We examine the role of resonant coupled channel final state interactions
(FSI), as well as weak annihilation and exchange contributions in explaining
all the two body hadronic $D\rightarrow PP$ decay modes data. In the
un-unitarized amplitudes we include modified Wilson coefficients with
non-factorizable corrections as parameters. For the hadronic form factors, the
z-series expansion method is used to get the $q^2$ dependence. The FSI effects
are incorporated via a phenomenological approach with widths of resonances to
various channels taken from observations where available, and others as
additional parameters to be determined from fits of all the theoretical rates
to the measured ones. Our results for the rather hard to explain
$D^0\rightarrow K^+K^-, \pi^+\pi^- $ are in agreement with measured values. We
demonstrate that both weak exchange as well as FSI effects are required to get
the correct branching ratio for the $D^ \rightarrow K^0\bar{K^0}$ mode. Using
our unitarized amplitudes we evaluate the strong phase difference between the
amplitudes for $D^0\rightarrow K^-\pi^+$ and $D^0\rightarrow K^+\pi^-$ and find
it to be in complete agreement with the recent BES III result.

High Energy Physics - Phenomenology;High Energy Physics - Experiment