  A simple theoretical model describing the positive sign of subnatural-width
absorption resonances in the recent experiment of Akulshin and co-workers
(Phys. Rev. A, {\bf 57}, 2996 (1998)) is proposed. An analytical expression for
the linear response to the weak probe field is found in the low-saturation
limit with respect to the control field. It is shown that the positive sign of
subnatural resonance is caused by the spontaneous transfer of the light-induced
coherence from the excited level to the ground one.
