A "critical mass" is needed for a chain reaction because
A) if the mass is too small the temperature achieved during fission will be too low to sustain the reaction.
B) if the mass is too large, the temperature will get too high and the entire mass will blow up without fissioning.
C) if the mass is too large, the density will be too high to sustain a chain reaction.
D) if the mass is too small, too many neutrons will leak out without causing fission.
E) if the mass is too small, there will not be enough uranium to start the reaction.
D
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