Question ID:PT92 S3 Q3 Passage:Because most agricultural practices in North America produce row after row of only a few, genetically identical, varieties of crops, the continent's food system rests precariously on a rapidly eroding genetic base, increasingly susceptible to pests and disease. As a possible solution, agricultural researchers, development experts, and policy makers are searching for ways to conserve the wealth of genetic information contained in crops known as heirloom varieties, which have been developed and maintained by indigenous farmers for many generations. The store of agricultural knowledge associated with the cultivation of these varieties has only recently been recognized as a valuable resource by crop geneticists, who have tended to base their research and their thinking solely on agricultural methods brought to North America from elsewhere. But given the intensity with which these non‚ North American methods seek to maximize the land's output for economic reasons, they are not always suited to preserving a diverse pool of crop genetics.A recent study describes how generations of indigenous farmers relied on their understanding of practical genetics to develop hundreds of varieties of each indigenous plant cultivated. For example, long aware of the technique referred to now as hybridization, indigenous farmers frequently used the pollen from one variety of corn to fertilize another variety, thereby creating many varieties of sweet corn, dent or cereal corn, popcorn, and numerous other types. This variation allowed for the exploitation of the crops' natural ability to resist pests and disease. Furthermore, indigenous farmers maintained those varieties that provided the best yields in their local environment; varieties in the wet areas along the coastal plains, for instance, differed substantially from those grown in the mountains. Thus in addition to being more resistant to pests and disease, heirloom crops are less dependent on intensive irrigation systems because they have been selectively bred for particular environments.Many of these heirloom varieties are preserved in household seed stocks by indigenous farmers who obtain seeds through long-standing family, community, and regional exchange networks; similarly, knowledge of the required development and cultivation methods has been maintained through the centuries by intergenerational exchanges within an oral tradition. Over the past century, however, penetration of the world market economy into small agricultural communities, both native and non-native, has discouraged small-scale, internal food production in favor of income-generating work. The result has been a dwindling number of small farmers‚ with many of those planting nonheirloom, commercial seed varieties‚ and a breakdown of many community and regional exchange networks. These recent developments have made clear that immediate steps must be taken to preserve indigenous knowledge systems and integrate them with programs for the conservation of diverse crop genetics, thus bolstering the long-term sustainability of the continent's agricultural systems. Stem:Based on the passage, the expression "knowledge systems" (last sentence of the passage) most likely refers to which one of the following? Correct Answer Choice:DChoice A:sets of farming techniques developed by crop-genetics researchers Choice B:the passing of heirloom seed stock from generation to generation Choice C:techniques of hybridization brought to North America from elsewhere Choice D:methods for breeding and raising heirloom crop varieties Choice E:fail-safe techniques for successful farming operations