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Passage FiveAlthough no one is certain why migration occurs, there are several theories. One theory is based upon the premise that prehistoric birds of the northen Hemisphere were forced south during the Ice Age, when glaciers covered large parts of Europe, Asia, and North America. As the glaciers melted, the birds came back to their homelands, spent the summer, and then went south again as the ice advanced in winter. In time, the migration became a habit, and now, although the glaciers have disappeared, the habit continues.Another theory proposes that the ancestral home of all modern birds was the tropics. When the region became overpopulated, many species were crowded north. During the summer, there was plenty of food, but during the winter, scarcity forced them to return to the tropics.A more recent theory, known as photoperiodism, suggests a relationship between increasing daylight and the stimulation of certain glands in the birds’ bodies that may prepare them for migration. One scientist has been able to cause midwinter migrations by exposing birds to artificial periods of daylight. He has concluded that changes occur in the bodies of birds due to seasonal changes in the length of daylight. According to one theory, when the glaciers disappeared, birds （）
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目前我国各家商业银行推广的外汇结构化理财产品等都是( )。
Passage One"A rolling stone gathers no moss (苔藓) ," but there is one living animal that does gather moss, the three-toed sloth (树獭) of South America. This slowest-moving member of the animal kingdom is so inactive that moss actually gathers on its body and turns it green strange as it seems!Most of the sloth’s life is spent motionless, hanging upside down from a limb. And that is the way its hair grows. Long and coarse, the strands (串) from receptacles (花托) for the damp jungle algae (水藻) that turn the brown fur a mossy green. Actually this moss helps the animal survive because it serves as a per- fect camouflage against the leaf trees and hides the sloth from the jungle’s swift-moving hunters. The sloth would have little chance of survival on the ground. With long, curved claws hooked over the limb of a leafy tree, it spends the long hot hours during the day drowsing and eating. Inch by inch, it strips the leafy limbs bare and crawls Slowly down the trunk to find a new dining spot, but only at night.In addition to looking rather like a vegetable, the sloth is a strict vegetarian. Running out of its favorite leaf is about the only thing that will make a sloth move. Then its appetite may even force the animal into swimming a stream to reach a juicy succulent(多汁的值物). It will also force it into fighting to keep the tree all to itself. The sloth is lazy and prefers to be alone, but it will tolerate its own relatives! The author's purpose in writing the passage is（）
The phrase "let the American beef industry off the hook" ( Line 2, Para. 4 most probably means ______.
What result did the cow that had BSE produce
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