A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter $\frac{d}{2}$ in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively –

Question

( A ) $\frac{3f}{4} and \frac{I}{4}$
( B ) $f and \frac{I}{4}$
( C ) $\frac{f}{4} and \frac{I}{2}$
( D ) $f and \frac{3I}{4}$