Why Do Astronomers Consider Gamma-Ray Bursts to Be One of the Greatest

Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?

A) because they are so rare
B) because we know they come from pulsating variable stars but don't know how they are created
C) because the current evidence suggests that they are the most powerful bursts of energy that ever occur anywhere in the universe,but we don't know how they are produced
D) because current evidence suggests that they come from our own Milky Way,but we have no idea where in the Milky Way they occur
E) because current evidence suggests that they come from massive black holes in the centers of distant galaxies,adding to the mystery of black holes themselves