$A$ beam of light from a source $L$ is incident normally on a plane mirror fixed at a certain distance $x$ from the source. The beam is reflected back as a spot on a scale placed just above the source $L$. When the mirror is rotated through a small angle $\theta$,the spot of the light is found to move through a distance $y$ on the scale. The angle $\theta$ is given by

• A
  $\frac{y}{x}$
• B
  $\frac{x}{2y}$
• C
  $\frac{x}{y}$
• D
  $\frac{y}{2x}$