The graph shows how the magnification $m$ produced by a thin lens varies with image distance $v$. The focal length of the lens used is\\
(1) $\frac { b } { c }$\\
(2) $\frac { a } { c }$\\
(3) $\frac { b ^ { 2 } c } { a }$\\
(4) $\frac { b ^ { 2 } } { a c }$