Reactor "Meltdowns"like the One at Chernobyl in the Ukraine Were

Reactor "meltdowns"like the one at Chernobyl in the Ukraine were caused by

A) supercritical masses of fissile nuclides accumulating near the bottom of the reactor.
B) failure of the cooling system that distributes the heat generated in the reactor core.
C) decomposition of heavy water coolant D₂O into H₂O and neutrons that have very large thermal energies.
D) slow accumulation of critical masses of impurities at particular sites in the fuel rods, caused by improper design.
E) inhomogeneities in the containment magnetic fields caused by small temperature fluctuations in the field winding coils.