单选题 .  ①Attempts to identify New Guinean's hunter-gatherers face the well-known difficulty of defining what constitutes a hunter-gather group. ②According to the common definition, hunter-gathers are those who subsist by hunting wild animals and gathering wild plants. ③Yet those criteria beg numerous questions, including the issue of what constitutes "wild". ④The very presence on a landscape of humans who are consumers affects food resources, blurring the lines between wild and domesticated and, hence between hunting and pastoralism, and between gathering and cultivation. ⑤Moreover, it is unclear how groups should be classified that are hunter-gatherers in their procurement strategies but that make use of pastoralism and cultivation in their consumption patterns––subsisting, for example, by trading wild foods to neighbors in return for domesticated crops.15.  The primary purpose of the passage is to ______