单选题 A mysterious phenomenon is the ability of over-water migrants to travel on course. Birds, bees, and other species can keep track of 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, 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 the Earth by celestial navigation, almost as human navigators use stars and planets, but this would demand of 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.

单选题 The main idea of the passage is that ______.

单选题 It can be inferred from the passage that if the flock of birds described in the passage were navigating by compass sense alone, they would, after the storm, fly ______.

单选题 In maintaining that migrating animals would need "a fantastic map sense" to determine their geographic position by celestial navigation, the author intends to express ______.

单选题 Of the following descriptions of migrating animals, which most strongly suggests that the animals 'are depending on magnetic cues to orient themselves?