A beam of light is incident normally on a plane mirror fixed at a distance $x$ from a source. When the mirror is rotated through a small angle $\theta$, the reflected spot move through a distance $y$ on a scale. The angle $\theta$ is given by:

1. A
   $\frac{x}{y}$
2. B
   $\frac{x}{2y}$
3. C
   $\frac{y}{x}$
4. D
   $\frac{y}{2x}$