A concave mirror of radius of curvature R has a circular outline of radius r. A circular disc is to be placed normal to the axis at the focus so that it collects all the light that is reflected from the mirror from a beam parallel to the axis. For r << R, the area of this disc has to be at least
(A) πr6/4R4
(B) πr 4/4R2
(C) πr5/4R3
(D) πr4/R2
