Answer choice B is incorrect because the author of passage A never said that any/every action performed by early humans was to promote their genes' propagation. Did the author of passage A say that early humans never had fun or did something that wasn't required for passing on their genes? No, this would be going too far.
Answer choice C is correct because the author of passage B does in fact disagree that simply because a reason can explain a behaviour (e.g. passing on genes can explain altruism), doesn't make it so. Perhaps we can be altruistic, just because we care about others!