阅读理解
(3)According to some scientists, migratory birds should be able to withstand the winter. A bird's feathery coat is good insulation against the cold. Because a bird is warm-blooded, its body temperature always remains constant, even if the temperature of its surroundings changes. The factors that trigger migratory behavior in birds are difficult to explain. This behavior seems to be instinctive, not learned. For example many northern species leave their summer homes while the weather is still warm and the food supply plentiful. Young arctic terns born at the arctic breeding grounds will take off with the flock for distant lands they have never seen. Bird migrations are probably regulated by the glandular system. Scientists suspect that the changing length of the day is the factor that triggers migratory behavior. In an experiment, migratory birds were kept in artificially lighted rooms. It was found that if periods of darkness were lengthened proportionately, the glands of the birds became active. These glands secrete hormones, which are chemicals that control numerous body functions. Shorter periods of daylight seem to change the hormone balance of birds, so that they retain more fat. This stored fat that provides the energy for a long flight. The same experiment revealed that the birds became more excited as the artificial night was lengthened. It is probably no coincidence that most flocks begin their migratory flights during the night.
单选题12.According to the passage, which of the following protects birds against cold weather?
【正确答案】
C
【答案解析】根据文章第一段第二句“A bird’s feathery coat is good insulation against the cold．”可知，鸟类靠feather防寒，因此，正确答案为C。
单选题13.In the 2nd paragraph, the author mentions young arctic terns as an example of birds that______.
【正确答案】
C
【答案解析】文章第二段第二句提出鸟类迁徙的原因“This behavior seems to be instinctive，not learn—ed．”，并在下文中以young arctic terns为例论证这一观点。由此可知正确答案为C。
单选题14.According to the passage, birds exposed to longer periods of darkness experience all of the following changes EXCEPT______.
【正确答案】
D
【答案解析】文章最后一段第四句“It was found that if periods of darkness were lengthened proportion—ately，the glands of the birds became active．”，说明黑夜可以促进鸟类腺的运动，A项正确。“Shorter periods of daylight…they retain more fat．”，证明黑夜可以帮助鸟类贮存脂肪，B项正确。“The same experiment revealed that the birds became more excited asthe artificial night was lengthened．”，说明鸟类会因黑夜时间变长而变得激动，C项正确。因此，只有D项未提及。
单选题15.In the experiment mentioned in the passage, the scientists adjusted the birds'______.
【正确答案】
C
【答案解析】文章最后一段第三句“In an experiment，migratory birds were kept in artificially lightedrooms．”，点明这个实验改变了鸟类的光照条件。因此，C项正确。