With a concave mirror, an object is placed at a distance x₁ from the principal focus, on the principal axis. The image is formed at a distance x₂ from the principal focus. The focal length of the mirror is

• A

  x₁ x₂

• B

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

• C

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

• D

  $\sqrt{{\mathrm{x}}_{1 }{\mathrm{x}}_2}$