In a YDSE with $d= 1mm$ and $D= 1m$ slabs of $(t= 1 \mu m , \mu =3)$ and $(t=0.5\;\mu m , \mu =2) $ are introduced in front of upper and lower slit respectively . Then the shift in fringe pattern is :-

Question

(a) 3 mm upwards (b) 1 mm upwards (c)2 mm upwards (d)1.5 mm upwards