【单选题】

Mycorrhizal fungi infect more plants than do any other fungi and are necessary for many plants to thrive, but they have escaped widespread investigation until recently for two reasons. First, the symbiotic association is so well-balanced that the roots of host plants show no damage even when densely infecteD、Second, the fungi cannot as yet be cultivated in the absence of a living root.Despite these difficulties, there has been important new Work that suggests that this symbiotic association can be harnessed to achieve more economical use of costly superphosphate fertilizer and to permit better exploitation of cheaper, less-soluble rock phosphate. Mycorrhizal benefits are not limited to improved phosphate uptake in host plants. In legumes, mycorrhizal inoculation has increased nitrogen fixation beyond levels achieved by adding phosphate fertilizer alone.Certain symbiotic associations also increase the host plant’s resistance to harmful root fungi. Whether this resistance results from exclusion of harmful fungi through competition for sites, from metabolic change involving antibiotic production, or from increased vigor is undetermineD、

It can be inferred from the passage that which of the following has been a factor influencing the extent to which research on mycorrhizal fungi has progressedA．lack of funding for such research
B．lack of immediate application of such research
C．difficulties surrounding laboratory production of specimens for study.
D．lack of a method for identifying mycorrhizal fungi