Two solid spheres of identical fissile material have radii r and 2r, respectively. Neglecting neutron reflection from the surroundings, which statement most accurately compares their abilities to sustain a chain reaction?
(a) The larger sphere is more likely to be supercritical because its surface-area-to-volume ratio is one-half that of the smaller sphere, so fewer neutrons escape.
(b) Both spheres are equally likely to be supercritical because only the material composition, not the size, determines criticality.
(c) The smaller sphere is more likely to be supercritical because its neutron density is higher due to its smaller volume.
(d) The larger sphere is less likely to be supercritical because neutrons must travel farther before striking another nucleus, increasing the chance that they escape.