Identify the correct definition of "critical mass" for a fissile material.
a) The minimum amount of fissile material needed only to initiate a chain reaction but not sustain it
b) The mass of fissile material at which the neutron multiplication factor k just falls below 1
c) The minimum amount of fissile material required to sustain a self-maintaining fission chain reaction
d) The quantity of fissile material beyond which the chain reaction becomes uncontrollably explosive