Q 31. ​A tank contains a slab of glass 8 cm thick and of refractive index 1.6. Above this, is a depth of 4.5 cm of an oil and upon this a layer of water 6 cm thick and of refractive index 4/3. To an observer, looking down from above, a mark at the bottom appears to be raised upto position 6 cm from the bottom of the slab. The refractive index of oil is 
a) 0.8 
b) 1.2 
c) 1.5 
d) 1.8