Which of the following best defines the critical mass of a fissile material?
(a) The minimum mass required for the material to overcome electrostatic repulsion and undergo fusion.
(b) The minimum mass needed so that, on average, exactly one neutron from each fission causes another fission before escaping.
(c) The minimum mass needed so that, on average, at least two neutrons from each fission cause additional fissions before escaping.
(d) The mass at which half the nuclei have undergone radioactive decay in one half-life.