Why Doesn't Uranium Ore Spontaneously Undergo a Chain Reaction

Multiple Choice

Why doesn't uranium ore spontaneously undergo a chain reaction?

A) Uranium ore must be "weapons' grade" in order for it to undergo chain reaction.
B) Uranium ore is mostly non-fissionable isotope U-238 and undergoes a chain reaction only if mixed with a moderator to slow down the neutrons.
C) Uranium ore does spontaneously undergo slow chain reaction over time finally resulting in it becoming lead.
D) Uranium ore,mostly U-235,must be enriched with U-238 in order for it to undergo a chain reaction.