A convex lens of focal length \[f\] is placed somewhere in between an object and a screen. The distance between the object and the screen is \[x\]. If the numerical value of the magnification produced by the lens is \[m,\], then the focal length of the lens is

• A.            \[\frac{mx}{{{(m+1)}^{2}}}\]
• B.            \[\frac{mx}{{{(m-1)}^{2}}}\]
• C.            \[\frac{{{(m+1)}^{2}}}{m}x\]
• D.            \[\frac{{{(m-1)}^{2}}}{m}x\]