An image of sun is formed by a lens of focal length 30 cm on the metal surface of a photoelectric cell and it produces a current I. The lens forming the image is then replaced by another lens of  same diameter but of focal length 15 cm. The photocurrent in this case will be : (In both case  plate is kept at focal plane)

• A

  $\frac{I}{2}$

• B

  2 I

• C

  I

• D

  4 I