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 proteobacteria
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 protocell