Where Does the Energy Come from That Allows the Crab

Where does the energy come from that allows the Crab Nebula to keep shining almost a 1000 years after the star exploded? (Who ultimately "pays the energy bill"?)

A) material is still being ejected from the star in the form of a planetary nebula
B) a massive black hole is "eating" material at the center of the nebula
C) a neutron star is slowing down (losing rotation energy)
D) large parts of the nebula are falling inward, releasing gravi?tational energy
E) the Crab nebula is a signal station, where some aliens (LGM) are broad?casting beats for rap music