C. the geography and climate of the area where they settled.
One of the main ways in which the population of Western Europe changed after the collapse of the Roman Empire is that it became more diverse, since the official borders of the Empire were no more. There was also increased lawlessness. <span />
I vaguely remember reading the book. I remember it was in the perspective of a German infantryman. So i believe B would be the best answer. I recommend it, its a good book too.<span />
