According to the Endosymbiosis Theory of the Origin of Eukaryotic

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

A) through viral infection
B) from infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy- transfer reactions
C) from engulfed, originally free- living proteobacteria
D) by secondary endosymbiosis
E) from the nuclear envelope folding outward and forming mitochondrial membranes