A lens of focal $30\; cm$ and diameter 5 cm forms an image of intensity I. If central part of the lens is covered by black circular paper of diameter $2.5 \;cm$ What will be the new focal length and new intensity of image.

Question

$(a)\;15\;cm,I \\ (b)\;30\;cm,\frac{I}{4} \\ (c)\;15\;cm, \frac{3I}{4} \\ (d)\;30\;cm, \frac{3I}{4}$