According to the Endosymbiotic Theory of the Origin of Eukaryotic

According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate?

A) from infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy-transfer reactions
B) from engulfed, originally free-living prokaryotes
C) by secondary endosymbiosis
D) from the nuclear envelope folding outward and forming mitochondrial membranes
E) when a protoeukaryote engaged in a symbiotic relationship with a protobiont