C. Rotating black holes and the Blandford Znajek mechanism
It is possible and even likely that the process of energy extraction involves the Blandford-Znajek mechanism  in which the black hole - torus system is engulfed in a magnetic field and the rotational energy of the black hole is extracted via this magnetic field. The exploration of the Blandford-Znajek mechanism involves relativistic MHD consideration which are beyond the scope of this review. I refer the reader to several recent extensive reviews on this subject (see e.g. Lee et al. ).