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.

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