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Because fathers were able to use their hands to gather food and carry it to their mates from a distance. This allowed mothers to expend greater amounts of time and energy to the nurture and protection of their children.
This implies that before this development mothers had to spend more time acquiring their own food, and therefore less time nurturing and protecting their offspring.
Walking upright permitted the father to use his hands to gather food and carry it to his mate from a distance, allowing the mother to devote more time and energy to nurturing and protecting their children. 这里说直立行走允许父亲使用他的手来收集食物并且从较远距离带回给他们的配偶，进而允许母亲投入更多的时间和精力陪养育和保护他们的孩子
这说明与原来四只脚走相比的时候，她们有更多的时间和精力，说明原来的时候母亲也会去找食物
问的是在直立行走的转变发生前的情况。直立行走后男鞋可以带事物给女性，女性专心带孩子。allowing the mother to devote more time and energy to nurturing and protecting their children. more说明直立行走前没那么时间和精力花在带小孩上。
问的是直立行走的转变发生前的情况，你说的是直立行走后的。allowing the mother to devote more time and energy to nurturing and protecting their children. more说明之前要更少。
有可能选项完全正确，但因不满足题干的各种限制而排除。