According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate?
A. From engulfed, originally free-living bacteria
B. When a protoeukaryote engaged in a symbiotic relationship with a protocell by secondary endosymbiosis
C. From the nuclear envelope folding outward and forming mitochondrial membranes
D. From infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy-transfer reactions