  We compute the effects of thermal Comptonization of soft photons emitted from
a Kep- lerian disk around a black hole by the post-shock region of a
sub-Keplerian flow, known as the CENtrifugal pressure dominated BOundary Layer
(CENBOL). We show that the spectral state transitions of black hole candidates
could be explained either by varying the outer boundary of the CENBOL, which
also happens to be the inner edge of the Keplerian disk, or by changing the
central density of the CENBOL which is governed by the rate of the
sub-Keplerian flow. We confirm the conclusions of the previous the- oretical
studies that the interplay between the intensity of the soft photons emitted by
the Keplerian flow and the optical depth and electron temperature of the
Comptonizing cloud is responsible for the state transitions in a black hole.
