With a concave mirror, an object is placed at a distance $x_{1}$ from the principal focus, on the principal axis. The image is formed at a distance $x_{2}$ from the principal focus. The focal length of the mirror is
(a) $x_{1} x_{2}$
(b) $\frac{x_{1}+x_{1}}{2}$
(c) $\sqrt{\frac{x_{1}}{x_{2}}}$
(d) $\sqrt{x_{1} x_{2}}$