Which of the Following Statements Best Describes a Critical Mass

Which of the following statements best describes a critical mass?

A) A critical mass is the minimum amount of material that will undergo nuclear detonation.
B) A critical mass is the minimum amount of material that will spontaneously undergo a chain reaction.
C) A critical mass cannot be assembled from smaller,noncritical pieces of material.
D) A critical mass will detonate when it is separated into subcritical pieces.
E) none of the above