In Order for a Substance to Sustain a Fission Chain

In order for a substance to sustain a fission chain reaction, one important condition is that

A) the nuclei must collide with enough energy to overcome the electrical repulsion between them.
B) energy must be continually supplied from the outside.
C) each fissioning nucleus must release a sufficient number of alpha- particles.
D) each fissioning nucleus must release a sufficient number of neutrons.
E) the nuclei must stay in shape by maintaining a regular jogging program.