A beaker containing liquid is placed on a table, underneath a microscope which can be moved along a vertical scale. The microscope is focused, through the liquid onto a mark on the table when the reading on the scale is `a`. It is next focused on the upper surface of the liquid and the reading is `b`. More liquid is added and the observations are repeated, the corresponding readings are `c` and `d`. The refractive index of the liquid is

Options

A. `(d-b)/(d-c-b+a)`
B. `(d-c-b+a)/(d-b)`
C. `(b-d)/(d-c-b+a)`
D. `(d-c-b+a)/(b-d)`