A thin lens focal length `f_(1)` and its aperture has diameter `d`. It forms an image of intensity `I`. Now the central part of the aperture up to diameter`(d)/(2)` is blocked by an opaque paper. The focal length and image intensity will change to
A. `f//2, l//2`
B. `f, l//4`
C. `3f//4, l//2`
D. `f, 3l//4`