Displacement method is applicable to determine the focal length of convex lens. In this method, the applicable formula is `f= (D^(2) - X^(2) )/( 4D)`. Here `D=` distance between object plane and screen on which image is formed `f=` focal length of lens, `X=` distance between two position of lens. To determine focal length of lens, the measure value of D and x are 90.0cm and 30.0cm, respectively. If percentage error in measurement of focal length is `n xx 10^(-1)%`. Find the value of n.

Options

A. `40 cm`
B. `10 cm`
C. `30 cm`
D. `20 cm`

Answer

D