The Explosions at the Fukushima Nuclear Power Plant in Northeast

The explosions at the Fukushima Nuclear Power Plant in northeast Japan following damage by an offshore earthquake blew the roofs off of three of the reactor buildings. The explosions were caused

A) presumably by a meltdown of the reactor cores.
B) presumably by steam from the heat exchangers inside the reactor cores
C) presumably by a buildup of hydrogen gas when the backup generators were disabled
D) terrorists taking advantage of the chaos of the situation
E) spent fuel rods that were exposed when the storage water tanks cracked and stopped cooling the rods