A chimp who displays feelings of affection toward the other members of its social group is more likely to be defended by these group members from raiders outside of the group -- even at the risk of harm to these defenders -- than are those chimps who rarely or never display feelings of affection toward their associates. This shows that, from a sociological perspective, affection plays the same role in chimp communities as in human communities, since humans are more willing to face risks to protect those toward whom they have feelings of affection. Which one of the following is an assumption on which the argument depends? ['Expression of affection in chimps is limited to members of the social group to which they belong.', 'Feelings of affection are the only reason humans protect each other.', 'Chimps express their emotions behaviorally whenever they feel them.', 'Feelings of affection in chimp communities are at least sometimes reciprocated.']