What contribution to atomic theory resulted from Albert Einstein's work? O A. A new understanding of what caused some elements to be
radioactive
B. A new model of the atom that described electrons as being in a
cloud
C. The discovery that different numbers of neutrons produced
isotopes
O D. A new understanding that the nucleus contained protons and
neutrons

a is the correct answer

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.