The distance of an object from first focal point of a biconvex lens is $X_1$ and distance of the image from second focal point is $X_2$. The focal length of the lens is (A) $X_1\,X_2$ (B) $\sqrt{X_1 + X_2}$ (C) $\sqrt{X_1\,X_2}$ (D) $\sqrt{\frac{X_2}{X_1}}$