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 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.

单选题 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.

单选题 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 progressed?
A. 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.

单选题 The level of information in the passage above is suited to the needs of all of the following people except ______.
A. an official of a research foundation who identifies research projects for potential funding
B. a state official whose position requires her to alert farmers about possible innovations in fanning
C. a researcher whose job is to identify potentially profitable areas for research and product development
D. a botanist conducting experiments to determine the relationship between degree of mycorrhizal infection and expected uptake of phosphate

单选题 The passage suggests which of the following about the increased resistance to harmful root fungi that some Plants infected with mycorrhizal fungi seem to exhibit?
A. 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 increase.