According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate? a) when a protoeukaryote engaged in a symbiotic relationship with a protocell

b) from infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy-transfer reactions

c) from the nuclear envelope folding outward and forming mitochondrial membranes

d) by secondary endosymbiosis

e) from engulfed, originally free-living proteobacteria