单项选择题

　　The Headland Hypothesis argues that foraging or non-agricultural tribes　have been unable to collect adequate carbohydrates in the rain forest due to its lack of starch producing species, and were thus forced to develop trade relationships with agriculturalists. This hypothesis has been shown to rest on impossibly idealized conceptions of virgin rain forest, forager behavior and history, such that one may argue something diametrically different: millennia of trade relationships with agricultural peoples have led to changes in forager behaviors and in the composition of the forests they inhabit. Supposing that humans modify their environments in ways that are generally favorable toward their continued survival, it follows that an increased reliance on agriculturalists for carbohydrates might lead to the gradual disappearance of rain forest starches. Horticulturalists are likely to dedicate the majority of their efforts toward staple starch crops such as rice or wheat, which in some environments may provide a more efficient source of carbohydrates than does foraging. Foragers, then, would be inclined to assume the "professional primitive" role, and trade more tasty and nutritious rain forest resources such as meat and fruit in exchange for carbohydrates, as Headland himself observed in a multitude of cultures around the world. Foragers may have also lost some of their knowledge and technologies related to carbohydrate extraction from the rain forest, and the carbohydrate-rich rain forest species may have arrested their co-evolution with foragers, leaving the impression that rain forests have always possessed insufficient quantities of such resources to support humans. A co-evolutionary argument is not, however, necessary to this line of reasoning, for rain forests may adapt purely in terms of the quantity and availability of extant carbohydrate-rich species, as the case of sago palms evinces in two ways. Firstly, the selective harvesting of some trees has been shown to have a "thinning" effect which helps the species to gain sunlight and to thrive, positively affecting its long-term survival, reproduction and distribution at the expense of carbohydrate-rich species. Secondly, the sago palm has two means of reproduction: vegetatively, or through "suckers", and through seed disbursal, which whether intentional and inadvertent is likely to increase when humans are harvesting the trees. Although sago palms are particularlv nrevalent in the areas where, for instance the Penan foragers exploit it, there has been no study to show that this would remain the case if the Penan were to move, or to cease exploiting the trees.
　　Admittedly, this response to the Headland Hypothesis has problems, for not all carbohydrate producing species are disbursed by seeds, nor have they all been shown to benefit from human foraging behaviors. Theories of co-evolution do, however, predict that such relationships would be likely to evolve, and the simple fact that disturbing the rain forest through fire, sago harvesting, and　countless other means available to foragers can lead to better environments for carbohydrate growth, illustrates that significant changes could have occurred in much less time than one might expect.

The author of the passage uses the second paragraph to（）.

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