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