Passage 1
Plants and animals that have been studied carefully seem to have built-in clocks. These biological clocks, as they are called, usually are not quite exact in measuring time. However, they work pretty well because they are "reset" each day, when the sun comes up.
Do pigeons use their biological clocks to help them find directions from the sun? We can keep pigeons in a room lit only by lamps. And we can program the lighting to produce artificial "days",different from the day outside. After a while we have shifted their clocks. Now we take them far away from home and let them go on a sunny day. Most of thegn start out as if they know just which way to go, but choose a wrong direction. They have picked a direction that would be correct for the position of the sun and the time of day according to their shifted clocks.
It is known and experimented that homing pigeons can tell directions by the sun. But what happens when the sky is darkly overcast by clouds and no one can see where the sun is? Then the pigeons still find their way home. The salne experiment has been repeated many times on sunny days and the result was always the same. But on very overcast days clock-shifted pigeons are just as good as normal pigeons in starting out in the right directions. So it seems that pigeons also have some extra sense of direction to use when they cannot see the sun.
Naturally, people have wondered whether pigeons might have a built-in compass—something that would tell them about the directions of the earth's magnetic field. One way to test that idea would be to see if a pigeon's sense of direction can be fooled by a magnet attached to its back.
With a strong magnet close by, a compass can no longer tell direction.
To test the idea, a group of ten pigeons had strong little magnet bars attached to their backs.
Another group carried brass bars instead which were not magnetic. In a number of experiments,both groups were taken away from home and let go. On sunny days none of the magnet-pigeons was fooled. They were just as good as the brass-pigeons in starting out in the right direction toward home. On cloudy, overcast days, however, with no sun the brass-pigeons chose the right direction,but the magnet-pigeons were in trouble. They later started out in different directions and acted completely lost.

What does the author want to say by citing the last experiment?
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A．On heavily cloudy days, pigeons that are not well trained do not fly.
B．Pigeons use different sources to find directions on different weather.
C．Magnetic field is an important source in helping pigeons to find directions.
D．The little magnet bars make it easier for pigeons to find directions.