In a cave mirror experiment, an object is placed at a distance x₁ from the focus and the image is formed at a distance x₂  from the focus. The focal length of the mirror would be

• A

  ${\mathrm{x}}_1{\mathrm{x}}_2$

• B

  $\sqrt{{\mathrm{x}}_1{\mathrm{x}}_2}$

• C

  $\frac{{\mathrm{x}}_1+{\mathrm{x}}_2}{2}$

• D

  $\sqrt{\frac{{\mathrm{x}}_1}{{\mathrm{x}}_2}}$