  We present measurements of production cross sections of single top quarks in
$\ppbar$ collisions at $\sqrt{s}=1.96\;\rm TeV$ in a data sample corresponding
to an integrated luminosity of $5.4\;\rm fb^{-1}$ collected by the D0 detector
at the Fermilab Tevatron Collider. We select events with an isolated electron
or muon, an imbalance in transverse energy, and two, three, or four jets, with
one or two of them containing a bottom hadron. We obtain an inclusive cross
section of $\sigma({\ppbar}{\rargap}tb+X, tqb+X) = 3.43\pm^{0.73}_{0.74}\;\rm
pb$ and use it to extract the CKM matrix element $0.79 < |V_{tb}| \leq 1$ at
the 95% C.L. We also measure $\sigma({\ppbar}{\rargap}tb+X) =
0.68\pm^{0.38}_{0.35}\;\rm pb$ and $\sigma({\ppbar}{\rargap}tqb+X) =
2.86\pm^{0.69}_{0.63}\;\rm pb$ when assuming, respectively, $tqb$ and $tb$
production rates as predicted by the standard model.
