It affects populations. It affects only individuals.
It occurs when there is genetic variation.
It occurs when organisms are genetically identical.
It selects organisms with certain traits to survive.
It selects organisms at random to survive. Explain how natural selection can lead to evolution.

i believe the answer is a.

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.