How Do We Know That the Compact Object in Some

How do we know that the compact object in some X-ray binaries must be a black hole?

A) X-rays of high enough energy can be emitted only by a black hole.
B) The orbital period of the binary requires the compact object to have more than 1.4 solar masses.
C) The orbital period of the binary requires the compact object to have more than 3 solar masses.
D) The compact object does not produce any X-rays.