  We propose an effective single-band model for the newly discovered iron
selenide superconductors A$_x$Fe$_{2-y}$Se$_2$ (A=Tl,K,Rb,Cs). Based on this
minimum model and the random phase approximation, the $d_{x^2-y^2}$ pairing
symmetry is revealed theoretically, which may be understood in the framework of
Fermi surface topology. A common origin of superconductivity is elucidated for
this compound and other high-T$_c$ materials. The spin excitations at
$(\pi,\pi/2)$ in superconducting states are observed, in good agreement with
the neutron scattering experiments. The spin resonance is indicated to show up
only for the d-wave pairing, which provides an additional indication for the
d-wave pairing symmetry in this family of superconductors.
