To make the central fringe at the center`O` , mica sheet of refractive index `1.5` is introduced Choose the corect statement.
A. the thickness of sheet is `2(sqrt(2)-1)d` infront of `S_(1)`
B. the thickness of sheet is `(sqrt(2)-1)d` infront of `S_(2)`
C. the thickness of sheet is `2sqrt(2)d` infront of `S_(1)`
D. the thickness of sheet is `(2sqrt(2)-1)d` infront `S_(1)`.