  Careful discussions are made on some points which are met in studying B decay
final state interactions, taking the $B^0\to \pi^+ K^-$ process as an example.
We point out that $\pi$--exchange rescatterings are not important, whereas for
$D^*$ and $D^{**}$ exchanges, since the $B^0\to D^+D_s^-$ decay has a large
branching ratio their contributions may be large enough to enhance the $B\to
\pi K$ branching ratio significantly.
  However our estimates fail to predict a large enhancement.
