Why is X-gal used in cloning medium?

A) It distinguishes between bacteria that have been transformed with recombinant plasmids and nonrecombinant plasmids by black-white screening.
B) It distinguishes between bacteria that have been transformed with recombinant plasmids and nonrecombinant plasmids by blue-white screening.
C) It distinguishes between plasmids that have been transformed and those that have not by blue-white screening.
D) It distinguishes between plasmids that have been transformed and those that have not by black-white screening.