In Fission Bombs, a Certain "Critical Mass" Is Needed Because

In fission bombs, a certain "critical mass" is needed because

A) the fissionable material will not reach a sufficiently high temperature unless this much material is present.
B) too small an amount of uranium will not hold together throughout the reaction, and thus the uranium falls apart and the reaction goes out before it is completed.
C) for smaller masses the pressure is so low that neutrons move too slowly through the uranium to sustain a chain reaction.
D) if the mass is too small, too many neutrons pass through the uranium and out the sides without hitting a nucleus.
E) it is impossible to fission a uranium nucleus unless it is near a large number of other uranium nuclei.