Directions: In this part of the test, there are five passages. After each passage, there are five questions followed by four choices marked A, B, C and D. Choose the best answer and then write the corresponding letter on your Answer Sheet.

Passage One

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 them 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 same 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