When a thin film F1 is pasted in front of upper slit of YDSE experiment,the fringe pattern shifts up by 1 mm. When another thin film F2 is pasted in front of lower slit, the fringe pattern comes back to it's original position.
(A) The two films are equally thick
(B) The optical path of light in both films is equal
(C) The refractive index of both films are equal
(D) None of these
