The focal length of a lens is expressed by which of the following:

Options

A. `frac(1)(f)= frac(1)(u) + frac(1)(v)`
B. `frac(1)(f)=frac(1)(v)-frac(1)(u)`
C. `frac(1)(f)=frac(1)(u)=frac(1)(v)`
D. `frac(1)(f)=frac(1)(u)-(1)(u)`