I vote for C too.
Never good at explanations, I attempt thus ...
A and B are awkward.
D kind of suggests that 'because' they were ignorant .. they lived like that.
E says that 'each' was ignorant of all others' exixtence. We need 'the' to talk about someone specific viz, Europeans/Native Americans.
God!! I need to be more technical with my explanations I guess. Isn't 'gut feeling' a good enough explanation?