Natural uranium fails to sustain a fast-neutron chain reaction primarily because
a) fast neutrons must first be slowed down; their fission probability in 235U is negligible while they are fast
b) the number of neutrons released per fission in natural uranium is less than one
c) most fast neutrons are captured by 238U before they can cause fission
d) because natural uranium has a lower density, many neutrons escape without interacting