单选题 According to some scientists, migratory (迁徙的) birds should be able to withstand (抵抗) the winter. A bird's feathery coat is a good protection against the cold. Because a bird is warm-blooded, its body temperature always remains unchanging, even if the temperature of its surroundings (周围环境) changes.
The factors (因素) that bring about migratory behavior (行为) in birds are difficult to explain. It seems that this kind of behavior is not be learned but inborn. For example, many northern birds leave their summer homes while the weather is still warm and there is still enough food to eat.
Bird migrations are probably regulated (调整) by the glandular system (腺性分泌系统). Scientists think that the changing length of the day is the factor that brings about migratory behavior. In an experiment, migratory birds were kept in artificially (人工地) lighted rooms. It was found that if periods of darkness were lengthened proportionately (按比例地), the glands of the birds became active. These glands {{U}}secrete{{/U}} hormones (荷尔蒙), which are chemicals that control lots of body functions (机能). Shorter periods of daylight seem to change the hormone balance of birds, so that they retain more fat. This stored fat is the fuel that provides the energy for a long flight. The same experiment showed that the birds became more excited as the artificial night was lengthened. It is probably no coincidence (巧合) that most birds begin their migratory flights during the night.
单选题 What does the passage mainly discuss?
  • A. Common migratory paths for birds.
  • B. Why birds migrate.
  • C. Birds that do not migrate.
  • D. Migration in cold winter.
【正确答案】 B
【答案解析】[解析] 主旨题。题干意为“文中主要讨论了什么?”通读全文我们知道，文章主要介绍了科学家把一些候鸟饲养在有人工照明的屋内进行一项实验，从而来证明他们的想法：候鸟的迁移有可能是受到有规律的腺性分泌系统的影响。因此本题的正确答案为B。
单选题 According to the passage, which of the following protects birds against cold weather?
  • A. Glands.
  • B. Hormones.
  • C. Feathers.
  • D. Artificial light.
【正确答案】 C
【答案解析】[解析] 细节题。题干意为“根据文章的描述，什么可以保护鸟儿不受严冬的侵袭?”从文章第一段第二句话“A bird's feathery coat is a good protection against the cold.”可知，本题的正确答案为C。
单选题 In the experiment the scientists changed ______.
  • A. the birds' food supply
  • B. the birds' body temperatures
  • C. the birds' exposure (暴露) to light
  • D. the birds' brain chemistry
【正确答案】 C
【答案解析】[解析] 推断题。题干意为“在实验中科学家改变了______。”从文章最后一段第二句和第三句话“scientists think that the changing length of the day is the factor that brings about migratory behavior. In an experiment, migratory birds were kept in artificially lighted rooms.”可推知，在实验中科学家改变了照明时间。所以本题的正确答案为C。
单选题 The underlined word "secrete" (line 5, para.3) is closest in meaning to ______.
  • A. produce
  • B. hidden
  • C. borrow
  • D. hide
【正确答案】 A
【答案解析】[解析] 语义题。题干意为“与第三段第五行‘secrete’这个单词的意思最接近的 单词是______。”从文章第三段第五句话“These glands secrete hormones, which are chemicals that control lots of body functions.”可知，这些腺体产生荷尔蒙，而荷尔蒙是可以控制身体许多功能的化学物质。所以本题的正确答案为A。