A monochromatic beam of light fall on YDSE apparatus at some angle (say `theta`) as shown in figure. A thin sheet of glass is inserted in front of the lower slit `s_(2)`. The central bright fringe (path difference `= 0`) will be obtained
A. at O
B. above O
C. below O
D. anywhere depending on angle `theta`, thickness of plate t and refractive index of glass `mu`