  In the mean field approximation of (2+1)-flavor Nambu--Jona-Lasinio model, we
strictly derive several sets of coupled equations for the chiral
susceptibility, the quark number susceptibility, etc. at finite temperature and
quark chemical potential. The critical exponents of these susceptibilities in
the vicinity of the QCD critical end point (CEP) are presented in SU(2) and
SU(3) cases, respectively. It is found that these various susceptibilities
share almost the same critical behavior near the CEP. The comparisons between
the critical exponents for the order parameters and the theoretical predictions
are also included.
