A mysterious phenomenon is the ability of over-water migrants to travel on course. Birds, bees, and other species can keep track of (keep track of: v.明了) time without any sensory cues from the outside world, and such “biological clocks” clearly contribute to their “compass sense.” For example, they can use the position of the Sun or stars, along with the time of day (the time of day: n.时刻), to find north. But compass sense alone cannot explain how birds navigate the ocean: after a flock traveling east is blown far south by a storm, it will assume the proper northeasterly course to compensate. Perhaps, some scientists thought, migrants determine their geographic position on Earth by celestial navigation, almost as human navigators use stars and planets, but this would demand of (demand of: v.要求) the animals a fantastic map sense. Researchers now know that some species have a magnetic sense, which might allow migrants to determine their geographic location by detecting variations in the strength of the Earth’s magnetic field.
17. The main idea of the passage is that
(A) migration over land requires a simpler explanation than migration over water does
(B) the means by which animals migrate over water are complex and only partly understood
(C) the ability of migrant animals to keep track of time is related to their magnetic sense
(D) knowledge of geographic location is essential to migrants with little or no compass sense（B）
(E) explanations of how animals migrate tend to replace, rather than build on, one another
18. It can be inferred from the passage that if the flock of birds described in line 4 were navigating by compass sense alone, they would, after the storm, fly
19. In maintaining that migrating animals would need “a fantastic map sense” (line 17) to determine their geographic position by celestial navigation, the author intends to express
(A) admiration for the ability of the migrants
(B) skepticism about celestial navigation as an explanation
(C) certainly that the phenomenon of migration will remain mysterious
(D) interest in a new method of accounting for over-water migration（B）
(E) surprise that animals apparently navigate in much the same way that human beings do
20. Of the following descriptions of migrating animals, which most strongly suggests that the animals are depending on magnetic cues to orient themselves?
(A) Pigeons can properly readjust their course even when flying long distances through exceedingly dense fogs.
(B) Bison are able to reach their destination by passing through a landscape that has been partially altered by a recent fire.
(C) Elephants are able to find grounds that some members of the herd have never seen before.
(D) Swallows are able to return to a given spot at the same time every year.（A）
(E) Monarch butterflies coming from different parts of North America are able to arrive at the same location each winter.
Mycorrhizal fungi (mycorrhizal fungi: 菌根真菌) infect more
Mycorrhizal fungi (mycorrhizal fungi: 菌根真菌) 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 (as yet: adv.至今) 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 (superphosphate fertilizer: 过磷酸钙肥料) 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 (phosphate fertilizer: 磷肥(料)) 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.
17. Which of the following most accurately describes the passage?
(A) A description of a replicable experiment
(B) A summary report of new findings
(C) A recommendation for abandoning a difficult area of research
(D) A refutation of an earlier hypothesis（B）
(E) A confirmation of earlier research
18. The level of information in the passage above is suited to the needs of all of the following people EXCEPT:
(A) a researcher whose job is to identify potentially profitable areas for research and product development
(B) a state official whose position requires her to alert farmers about possible innovations in farming
(C) an official of a research foundation who identifies research projects for potential funding
(D) a biologist attempting to keep up with scientific developments in an area outside of his immediate area of specialization（E）
(E) a botanist conducting experiments to determine the relationship between degree of mycorrhizal infection and expected uptake of phosphate
19. 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) Lack of a method for identifying mycorrhizal fungi
(D) Difficulties surrounding laboratory production of specimens for study（D）
(E) Difficulties ensuing from the high cost and scarcity of superphosphate fertilizers
20. 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) An explanation lies in the fact that mycorrhizal fungi increase more rapidly in number than harmful root fungi do.
(C) The plants that show increased resistance also exhibit improved nitrogen fixation.
(D) Such increases may be independent of mycorrhizal infection.（A）
(E) It is unlikely that a satisfactory explanation can be found to account for the increase.