$A$ ray of light is incident on a thick slab of glass of thickness $t$ as shown in the figure. The emergent ray is parallel to the incident ray but displaced sideways by a distance $d$. If the angles are small,then $d$ is

• A
  $t\left( {1 - \frac{i}{r}} \right)$
• B
  $rt\left( {1 - \frac{i}{r}} \right)$
• C
  $it\left( {1 - \frac{r}{i}} \right)$
• D
  $t\left( {1 - \frac{r}{i}} \right)$