zero Electric Resistance of the superconductors: why?

Superconductor definition:

Superconductors are the materials whose resistance become immeasurably small or actually  become zero below a critical temperature, T_c . Example- (La, Sr)₂ CuSO₄  with T_c= 36k.

Why does superconductor experience zero resistance?

This can be described according to BCS (which was developed in 1957 by Bardeen, Cooper, and Schrieffer) model. One key to the understanding of the BCS theory is accepting the existence of a pair of electrons (cooper pair) that has a lower energy than two individual electrons.

Imagine an electron in a metal with no lattice vibrations. This electron perturbs the lattice slightly in its neighborhood. When such an electron drifts through a crystal the perturbation is only momentary and after passing a displaced ion reverts back into its orginal position and oscillates around its rest position. So a phonon is created. This phonon in turn interacts quickly with a second electron which takes advantage of the deformation and lowers its energy. Electron two finally emits a phonon by itself which interacts with the first electron and so on. This passing back and forth of phonons couples the two electrons together and brings them into a lower energy state. One can visualize that all electrons on the Fermi-surface having opposite spin from those cooper pairs. Thus the superconducting state is an ordered state of the conduction electrons. The scattering on the lattice atoms is eliminated, thus causing a zero resistance