A beam of unpolarized light of intensity I_o is passed through a tourmaline crystal A and then through another tourmaline crystal B oriented so that its principal plane is parallel to that of A. If A is now rotated by 45'in a plane perpendicular to the direction of incident ray, the intensity of the emergent Iight will be

• A

  ${\mathrm{I}}_0/2$

• B

  ${\mathrm{I}}_0/\sqrt{2}$

• C

  ${\mathrm{I}}_0$

• D

  ${\mathrm{I}}_0/4$