Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 44.

Bird migration is one of the most interesting yet least understood natural phenomena. Every fall birds from northern latitudes fly in groups to the warmer southern latitudes and then return north in the spring. Scientists agree on the main reasons for migration: to follow the food supply and to avoid harsh climate conditions. For example, insects disappear during the cold months, prompting insect-dependent birds to fly south to warm areas where insects breed. No similar consensus has emerged, however, about how birds are able to navigate. Despite many recent experiments, bird experts still do not know how birds arrive at the same destinations every year and then find their way back home in the spring.

Some have suggested that birds find their way by following landmarks, such as rivers and mountain ranges. Experiments have confirmed that some species do follow such topographic features. But that method cannot explain how some birds travel at night. Other studies show that some nocturnal birds navigate by the stars. But that explanation cannot explain daytime migration or travel when the skies are cloudy.

The most popular explanation currently is that birds are guided by Earth’s magnetic poles. The mechanism by which that works has not yet been proved. One theory points to the fact that some birds’ contain magnetite, a naturally occurring magnetic compound consisting of iron oxide. Magnetite has been found in many animals, including birds. With magnets embedded in their brains, birds would be able to sense the magnetic fields of the North and South Poles.

A recent experiment with homing pigeons provided some evidence that magnetite does play a crucial role in migration. Homing pigeons are known to have the ability to return to their homes after being taken hundreds of miles away. Researchers found that they could train homing pigeons to recognize changes in a magnetic field. When a surrounding magnetic field was normal, the birds would gather at one end of a cage. But when the field’s polarity was altered, they hopped to the other end, suggesting that they were directing and responding to changes in the magnetic field.

Another theory has been offered to explain this sensitivity to magnetic poles, a theory that draws upon quantum mechanics, which is the study of how particles move inside an atom. It relies on the fact that electrons come in pairs that orbits the nucleus of an atom. The two electrons spin in opposite directions, creating two magnets that neutralize each other. But when molecules split and react with other molecules to form compounds, the electrons pairs may no longer spin in opposite directions. Instead, they may repel each other, as when two north ends of magnets are pressed together. The electrons struggle to change direction in order to achieve a stable state in which the two electrons again neutralize each other, giving off no magnetic field.

According to paragraph 3, birds can detect the magnetic fields of the North and South poles because

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