The focal length of a concave mirror is f and the distance from the object to the principle focus is x. The ratio of the size of the image to the size of the object is

• A

  $\frac{\mathrm{f}+\mathrm{x}}{\mathrm{f}}$

• B

  $\frac{\mathrm{f}}{\mathrm{x}}$

• C

  $\sqrt{\frac{\mathrm{f}}{\mathrm{x}}}$

• D

  $\frac{{\mathrm{f}}^2}{{\mathrm{x}}^2}$