  The QCD factorization (QCDF) method usually contains infrared divergences
which introduce large model dependence to its predictions on charmless B
decays. The amplitudes of charmless B decays can be decomposed into "tree" and
"penguin" parts which are conventionally defined, not from the topology of the
dominant diagrams, but through their associated CKM factors V_{ub}^* V_{uq} and
V_{tb}^* V_{tq}, respectively, with q=d,s. We find that for B_{d,s} \to \pi^+
K^- decays, the "tree" amplitude can be well estimated in QCDF with small
errors, as the endpoint singularities have been canceled to a large extent.
With this as the only input from QCDF and combined with flavor symmetry, the
branching ratio of B_s \to \pi^+ K^- are estimated to be significantly larger
than the CDF measurement. This contradiction could be solved if the form factor
F^{B_s K} is smaller than the light cone sum rules estimation or the "tree"
amplitude has been over estimated in QCDF. The latter possibility could happen
if charming penguins are nonperturbative and not small, as argued in soft
collinear effective theory. To differentiate between these two possibilities,
we examine the similar B_s \to \rho^+ K^- decay with the same technique. It is
found that a large part of the uncertainties are canceled in the ratio B(B_s
\to \rho^+ K^-)/B(B_s \to \pi^+ K^-). It is predicted to be 2.5 \pm 0.2 in QCDF
which is independent on the form factor. However if charming penguins are
important, this ratio could be very different from the QCDF prediction.
Therefore the ratio of these two branching ratios could be an interesting
indicator of the role of charming penguins in charmless B decays.
