The Material That Accretes onto a Neutron Star or Black

The material that accretes onto a neutron star or black hole is expected to emit X-rays because

A) the material will produce synchrotron radiation.
B) hydrogen nuclei begin to fuse and emit high-energy photons.
C) the material will become hot enough that it will radiate most strongly at X-ray wavelengths.
D) as the material slows down it converts thermal energy to gravitational potential energy.
E) none of the above.