Pulsating X-Ray Sources with Periods of a Few Seconds Are

Pulsating X-ray sources with periods of a few seconds are caused by:

A) the eclipsing of an X-ray-emitting star with a very hot surface by a cool companion in a close binary system.
B) the pulsation in radius, temperature, and hence luminosity of a hot Cepheid variable star with a surface temperature hot enough to emit X-rays.
C) matter falling violently onto the surface of a rotating neutron star from a close companion in a binary star system, causing an X-ray hot spot that disappears periodically behind the neutron star.
D) matter falling onto the surface of a very hot, rotating white dwarf from an ordinary companion star in a binary system, producing an X-ray-emitting hot spot that disappears periodically behind the white dwarf.