A solid sphere of 235U is cut into eight identical smaller spheres that are then separated by a few centimetres of air. Which of the following best explains why none of the small spheres can now sustain a chain reaction even though the original large sphere could?
(a) Cutting the sphere reduces the half-life of 235U, so fewer neutrons are produced each second to keep the reaction going.
(b) Most neutrons are absorbed by the surrounding air before they can reach another 235U nucleus in the small pieces.
(c) Each small sphere has a larger surface-area-to-volume ratio, so more neutrons escape without inducing additional fissions.
(d) Because the pieces are separated, the strong nuclear force between nuclei in different pieces is greatly reduced, preventing the reaction from propagating.