A scattering angle of means that an alpha particle will travels directly toward a nucleus, comes to a complete halt as it approaches the nucleus, and then turns around and starts moving directly away from the nucleus. At the time when the alpha particle comes to a complete halt, all of its kinetic energy will have been converted into potential energy. Therefore, fi we let r denote the distance between the alpha particle and nucleus at this moment, then
This can be easily solved for , yielding
Therefore, answer (B) is correct.