The Intense X-Rays Emitted by a Suspected Black Hole Are

The intense X-rays emitted by a suspected black hole are generated by what physical mechanism?

A) the blueshifting of light emitted by hot gas into the X-ray region of the electromagnetic spectrum by the extreme speed of the gas moving into the black hole
B) induced emission in atoms by the intense gravitational field as material moves toward the black hole
C) compressional heating as material moves into the hole
D) the deceleration of matter as it abruptly stops at the event horizon of the black hole