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
(A) east
(B) north
(C) northwest
(D) south（A）
(E) southeast
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.