Mycorrhizal fungi infect more plants than do any other fungi and are necessary for many pl

ants 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. De spite these difficulties, there has been important new work that suggests that this symbiotic association can be harnessed to achieve more economical use of costly super phosphate 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, mycorrhzal 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.

Which of the following most accurately describes the passage？

A．A description of a replicable experiment

B．A refutation of an earlier hypothesis

C．A summary report of new findings

D．A confirmation of earlier research