Read this excerpt from White Fang. Hated by his kind and by mankind, indomitable, perpetually warred upon and himself waging perpetual war, his development was rapid and one-sided. This was no soil for kindliness and affection to blossom in. Of such things he had not the faintest glimmering. The code he learned was to obey the strong and to oppress the weak. Grey Beaver was a god, and strong. Therefore White Fang obeyed him. But the dog younger or smaller than himself was weak, a thing to be destroyed.

Which best describes the force that influences White Fang’s code of conduct?

A: an external force based on other characters
B: an external force based on the wilderness setting
C: an internal force based on White Fang’s feelings
D: an internal force based on White Fang’s instincts