【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、

The passage suggests which of the following about the increased resistance to harmful root fungi that some plants infected with mycorrhizal fungi seem to exhibitA．There are at least three hypotheses that might account for the increase.
B．The plants that Show increased resistance also exhibit improved nitrogen fixation.
C．Such increases may be independent of mycorrhizal infection.
D．It is unlikely that a satisfactory explanation can be found to account for the increas