Which of the following best describes the endosymbiont theory?
a. mitochondria evolved from archaeal endosymbionts living within ancestral eukaryotic cells
b. mitochondria evolved from bacterial endosymbionts living within ancestral eukaryotic cells
c. chloroplasts evolved from bacterial endosymbionts living within ancestral archaeal cells
d. nucleus evolved from archaeal endosymbionts living within ancestral bacterial cells